A stellar occultation by Neptune's main satellite, Triton, was observed on 5 October 2017 from Europe, North Africa, and the USA. We derived 90 light curves from this event, 42 of which yielded a central flash detection. We aimed at constraining Triton's atmospheric structure and the seasonal variations of its atmospheric pressure since the Voyager 2 epoch (1989). We also derived the shape of the lower atmosphere from central flash analysis. We used Abel inversions and direct ray-tracing code to provide the density, pressure, and temperature profiles in the altitude range $\sim$8 km to $\sim$190 km, corresponding to pressure levels from 9 {\mu}bar down to a few nanobars. Results. (i) A pressure of 1.18$\pm$0.03 {\mu}bar is found at a reference radius of 1400 km (47 km altitude). (ii) A new analysis of the Voyager 2 radio science occultation shows that this is consistent with an extrapolation of pressure down to the surface pressure obtained in 1989. (iii) A survey of occultations obtained between 1989 and 2017 suggests that an enhancement in surface pressure as reported during the 1990s might be real, but debatable, due to very few high S/N light curves and data accessible for reanalysis. The volatile transport model analysed supports a moderate increase in surface pressure, with a maximum value around 2005-2015 no higher than 23 {\mu}bar. The pressures observed in 1995-1997 and 2017 appear mutually inconsistent with the volatile transport model presented here. (iv) The central flash structure does not show evidence of an atmospheric distortion. We find an upper limit of 0.0011 for the apparent oblateness of the atmosphere near the 8 km altitude.

This paper presents the results of a combined spectroscopic and photometric study of 20 contact binary systems: HV Aqr, OO Aql, FI Boo, TX Cnc, OT Cnc, EE Cet, RW Com, KR Com, V401 Cyg, V345 Gem, AK Her, V502 Oph, V566 Oph, V2612 Oph, V1363 Ori, V351 Peg, V357 Peg, Y Sex, V1123 Tau, and W UMa, which was conducted in the frame of the W UMa Project. Together with 51 already covered by the project and an additional 67 in the existing literature, these systems bring the total number of contact binaries with known combined spectroscopic and photometric solutions to 138. It was found that mass, radius, and luminosity of the components follow certain relations along the MS and new empirical power relations are extracted. We found that 30 per cent of the systems in the current sample show extreme values in their parameters, expressed in their mass ratio or fill-out factor. This study shows that, among the contact binary systems studied, some have an extremely low mass ratio (q < 0.1) or an ultrashort orbital period (Porb < 0.25 d), which are expected to show evidence of mass transfer progress. The evolutionary status of these components is discussed with the aid of correlation diagrams and their physical and orbital parameters compared to those in the entire sample of known contact binaries. The existence of very short orbital periods confirms the very slow nature of the merging process, which seems to explain why their components still exist as MS stars in contact configurations even after several Gyr of evolution.

The observed light curves of V1241 Tau and GQ Dra in BVRI filters are given. Times of minimum light of V1241 Tau and GQ Dra as derived from our observations and TESS light curves are also listed. (2 data files).

Ultra-short orbital period contact binaries (Porb < 0.26 d) host some of the smallest and least massive stars. These systems are faint and rare, and it is believed that they have reached a contact configuration after several Gyrs of evolution via angular momentum loss, mass transfer and mass loss through stellar wind processes. This study is conducted in the frame of Contact Binaries Towards Merging (CoBiToM) Project and presents the results from light curve and orbital analysis of 30 ultra-short orbital period contact binaries, with the aim to investigate the possibility of them being red nova progenitors, eventually producing merger events. Approximately half of the systems exhibit orbital period modulations, as a result of mass transfer or mass loss processes. Although they are in contact, their fill-out factor is low (less than 30 per cent), while their mass ratio is larger than the one in longer period contact binaries. The present study investigates the orbital stability of these systems and examines their physical and orbital parameters in comparison to those of the entire sample of known and well-studied contact binaries, based on combined spectroscopic and photometric analysis. It is found that ultra-short orbital period contact binaries have very stable orbits, while very often additional components are gravitationally bound in wide orbits around the central binary system. We confirmed that the evolution of such systems is very slow, which explains why the components of ultra-short orbital period systems are still Main Sequence stars after several Gyrs of evolution.

Ultra-short orbital period contact binaries (Porb < 0.26 d) host some of the smallest and least massive stars. These systems are faint and rare, and it is believed that they have reached a contact configuration after several Gyrs of evolution via angular momentum loss, mass transfer and mass loss through stellar wind processes. This study is conducted in the frame of Contact Binaries Towards Merging (CoBiToM) Project and presents the results from light curve and orbital analysis of 30 ultra-short orbital period contact binaries, with the aim to investigate the possibility of them being red nova progenitors, eventually producing merger events. Approximately half of the systems exhibit orbital period modulations, as a result of mass transfer or mass loss processes. Although they are in contact, their fill-out factor is low (less than 30 per cent), while their mass ratio is larger than the one in longer period contact binaries. The present study investigates the orbital stability of these systems and examines their physical and orbital parameters in comparison to those of the entire sample of known and well-studied contact binaries, based on combined spectroscopic and photometric analysis. It is found that ultra-short orbital period contact binaries have very stable orbits, while very often additional components are gravitationally bound in wide orbits around the central binary system. We confirmed that the evolution of such systems is very slow, which explains why the components of ultra-short orbital period systems are still Main Sequence stars after several Gyrs of evolution.

We present a dynamical study of the intermediate polar and dwarf nova cataclysmic variable GK Persei (Nova Persei 1901) based on a multisite optical spectroscopy and R-band photometry campaign. The radial velocity curve of the evolved donor star has a semi-amplitude $K_2=126.4 \pm 0.9 \, \mathrm{km}\, \mathrm{s}^{-1}$ and an orbital period $P=1.996872 \pm 0.000009 \, \mathrm{d}$. We refine the projected rotational velocity of the donor star to $v_\mathrm{rot} \sin i = 52 \pm 2 \, \mathrm{km}\, \mathrm{s}^{-1}$ that, together with K2, provides a donor star to white dwarf mass ratio q = M2/M1 = 0.38 ± 0.03. We also determine the orbital inclination of the system by modelling the phase-folded ellipsoidal light curve and obtain i = 67° ± 5°. The resulting dynamical masses are $M_{1}=1.03^{+0.16}_{-0.11} \, \mathrm{M}_{\odot }$ and $M_2 = 0.39^{+0.07}_{-0.06} \, \mathrm{M}_{\odot }$ at 68 per cent confidence level. The white dwarf dynamical mass is compared with estimates obtained by modelling the decline light curve of the 1901 nova event and X-ray spectroscopy. The best matching mass estimates come from the nova light curve models and an X-ray data analysis that uses the ratio between the Alfvén radius in quiescence and during dwarf nova outburst.

Trans-Neptunian Objects (TNOs) have had an increasing interest since the discovery of (15760) Albion in 1992. These objects are considered remnants of the Solar System formation, and can thus provide clues about its origin and evolution. One of the best techniques to study TNOs from ground-based telescopes are stellar occultations which, if combined with photometric data, permit to obtain physical properties of the TNO such as its size, shape, and albedo. With this in mind, we predicted, observed, and analyzed the stellar occultation of the Gaia source 3444789965847631104 caused by the cubewano (19521) Chaos on the 20th of November 2020. The prediction was part of the searching program carried out by the Lucky Star project collaboration. We observed the object with the 1.23-m telescope at Calar Alto Observatory (Almería, Spain) two days before the event to update the prediction. The occultation observing campaign involved 19 observing stations, both professional and amateur, and resulted in three positive detections and 11 negative detections. Five teams could not observe due to bad weather. We fitted the positive chords' extremities to an ellipse to derive Chaos' projected size and shape and determine its geometric albedo. The preliminary area-equivalent diameter obtained is slightly smaller than the one derived with Herschel thermal data. However, we are still analyzing photometric data to complement and improve these results.

We invite observers to join an international observing campaign and obtain light curve of a selected number of the most ancient asteroids. Analysis of these data will be important to reconstruct the original state of the asteroid belt, which is a crucial problem of planetary science. This original state can be reconstructed through the identification of the oldest asteroid families. Traditional identification methods, like Hierarchical Clustering Methods (Zappala et al., 1990; HCM), have difficulties to recognise Gyr- and older asteroid families, whose members are very dispersed by the Yarkovsky effect. An innovative method, called V-shape search (Bolin et al., 2017), has been demonstrated effective (Deienno et al. 2021) to identify these aforementioned very old collisional families by searching for the signature of the size dependent dispersion of family members operated by the Yarkovsky effect.The method has already successfully identified two primordial families which likely formed before the giant planet orbital instability (Tsiganis et al., 2005) and could be as old as the Solar System and an ancient one that is ~3 Gyr-old (Delbo et al., 2017; 2019). There is evidence from observations and theoretical evolution models that there are more old families to be detected (Delbo et al., 2017; Dermott et al., 2018). However, the reliability of these V-shape families should be independently verified.A very important test of family membership is the anisotropic distribution of spin vectors of the asteroid family member, which is a fingerprint of the Yarkovsky effect evolution (Hanus et al. 2013). Namely, to test the working hypothesis that according to theories of asteroid orbital evolution under the Yarkovsky effect, members of the inward (outward) side of V-shape of a family have a statistical predominance of retrograde (prograde) objects (Fig. 1). This hypothesis has been tested (Hanus, et al., 2013) already for known families, and it is a reliable test for family membership. The observing campaign: Ancient AsteroidsFor this purpose, an international observing campaign called Ancient Asteroids has been put forward at different observatories worldwide: (University of Athens Observatory (UOAO), Greece, the BSA Observatory and Bigmuskie Observatory, Italy, the Observatoire de la Côte d'Azur (OCA), France, the Lowell Observatory in Arizona, United States, the Astronomical Institute of the Charles University in Prague, Czech Republic, the Aristotle University of Thessaloniki in Thessaloniki, Greece. The main goal of the campaign is to establish an international network of professional and amateur astronomers (Pro-Am collaboration), in order to perform photometric observations of a very specific sample of the most ancient asteroids. A special website (http://users.uoa.gr/~kgaze/research_asteroids_en.html) was developed for the purpose of the campaign, that includes guidelines for the participation, the observations and the image data evaluation and collection.In the frame of Ancient Asteroids campaign, a user-friendly web application was also developed for the target selection and observing plan preparation. The user can easily find the observable asteroids for the campaign based on the location, the time and the equipment limitations. The targets are displayed with a priority rate, in order to perform smart sampling. The user selects the favor target and an observing plan automatically is created. The contribution The obtained lightcurves from all the involved participants will be combined with data available in the literature, as well as with sparse data from space missions (Gaia, TESS, etc) and global sky surveys (PTF, LSST, ATLAS, etc). Thus, the spin state of the asteroids can be revealed. The results will be ingested to the Minor Planet Physical Properties Catalogue (MP3C) program (https://mp3c.oca.eu/catalogue/index.htm), the largest database of asteroid physical properties, which complies with the EU vision for open data. The observing campaign Ancient Asteroids has started to collect photometric observations with the contribution of amateur astronomers from Italy, France and Greece are submitted. The observations will reveal the spin state of the members, which are crucial for the testing the hypothesis of the family membership. This research will potentially lead to a better understanding of the first stages of the evolution of the Solar System, the mechanism at the origin of the formation of the asteroids and the planet formation processes. AcknowledgmentsThis work was also partially supported by the ANR ORIGINS (ANR-18-CE31-0014) and by the French National Program of Planetology (PNP). Here we make use of asteroid physical properties data from Minor Planet Physical Properties Catalog (https://mp3c.oca.eu/). ReferencesBolin, B. T. et al. (2017). Icarus, 282, 290-312.Deienno, R. et al. (2021). Icarus, 357, 114218.Delbo, M. et al. (2019). Astronomy & Astrophysics, 624, A69.Delbo, M. et al. (2017). Science, 357, 6355.Dermott, S. F. et al. (2018). Nature Astronomy, 2, 7.Hanus, J. et al. (2013). Astronomy & Astrophysics, 559.Hanus, J. et al. (2013). Astronomy & Astrophysics, 551.Tsiganis K. et al. (2005). Nature, 435, 7041.Zappala, V. et al. (1990). Astrophysical Journal, 100, 2030-2046.

The NASA Transiting Exoplanet Survey Satellite (TESS) observations consist of ~27 day long measurements with constant pointing to a 24°x96° field of view, called a "sector". In this paper, we present the first results on Cepheid stars observed with TESS, covering the first five out of 26 sectors of its primary 2yr near all-sky mission (2018-2019). (3 data files).

Binary and multiple stellar systems are numerous in our solar neighbourhood with 80 per cent of the solar-type stars being members of systems with high order multiplicity. The Contact Binaries Towards Merging (CoBiToM) Project is a programme that focuses on contact binaries and multiple stellar systems, as a key for understanding stellar nature. The goal is to investigate stellar coalescence and merging processes, as the final state of stellar evolution of low-mass contact binary systems. Obtaining observational data of approximately 100 eclipsing binaries and multiple systems and more than 400 archival systems, the programme aspires to give insights for their physical and orbital parameters and their temporal variations, e.g. the orbital period modulation, spot activity etc. Gravitational phenomena in multiple-star environments will be linked with stellar evolution. A comprehensive analysis will be conducted, in order to investigate the possibility of contact binaries to host planets, as well as the link between inflated hot Jupiters and stellar mergers. The innovation of CoBiToM Project is based on a multimethod approach and a detailed investigation, that will shed light for the first time on the origin of stellar mergers and rapidly rotating stars. In this work, we describe the scientific rationale, the observing facilities to be used and the methods that will be followed to achieve the goals of CoBiToM Project and we present the first results as an example of the current research on evolution of contact binary systems.

We predicted a stellar occultation of the bright star Gaia DR1 4332852996360346368 (UCAC4 385-75921) (mV = 14.0 mag) by the centaur 2002 GZ32 for 2017 May 20. Our latest shadow path prediction was favourable to a large region in Europe. Observations were arranged in a broad region inside the nominal shadow path. Series of images were obtained with 29 telescopes throughout Europe and from six of them (five in Spain and one in Greece) we detected the occultation. This is the fourth centaur, besides Chariklo, Chiron, and Bienor, for which a multichord stellar occultation is reported. By means of an elliptical fit to the occultation chords, we obtained the limb of 2002 GZ32 during the occultation, resulting in an ellipse with axes of 305 ± 17 km × 146 ± 8 km. From this limb, thanks to a rotational light curve obtained shortly after the occultation, we derived the geometric albedo of 2002 GZ32 (pV = 0.043 ± 0.007) and a 3D ellipsoidal shape with axes 366 km × 306 km × 120 km. This shape is not fully consistent with a homogeneous body in hydrostatic equilibrium for the known rotation period of 2002 GZ32. The size (albedo) obtained from the occultation is respectively smaller (greater) than that derived from the radiometric technique but compatible within error bars. No rings or debris around 2002 GZ32 were detected from the occultation, but narrow and thin rings cannot be discarded.

We present the first analysis of Cepheid stars observed by the TESS space mission in Sectors 1-5. Our sample consists of 25 pulsators: ten fundamental mode, three overtone and two double-mode classical Cepheids, plus three type II and seven anomalous Cepheids. The targets were chosen from fields with different stellar densities, both from the Galactic field and from the Magellanic System. Three targets have 2 minutes cadence light curves available by the TESS Science Processing Operations Center: for the rest, we prepared custom light curves from the full-frame images with our own differential photometric FITSH pipeline. Our main goal was to explore the potential and the limitations of TESS concerning the various subtypes of Cepheids. We detected many low-amplitude features: weak modulation, period jitter, and timing variations due to light-time effect. We also report signs of nonradial modes and the first discovery of such a mode in an anomalous Cepheid, the overtone star XZ Cet, which we then confirmed with ground-based multicolor photometric measurements. We prepared a custom photometric solution to minimize saturation effects in the bright fundamental-mode classical Cepheid, β Dor with the lightkurve software, and we revealed strong evidence of cycle-to-cycle variations in the star. In several cases, however, fluctuations in the pulsation could not be distinguished from instrumental effects, such as contamination from nearby sources, which also varies between sectors. Finally, we discuss how precise light-curve shapes will be crucial not only for classification purposes but also to determine physical properties of these stars.

Binary black hole (BH) central engine description for the unique blazar OJ 287 predicted that the next secondary BH impact-induced bremsstrahlung flare should peak on 2019 July 31. This prediction was based on detailed general relativistic modeling of the secondary BH trajectory around the primary BH and its accretion disk. The expected flare was termed the Eddington flare to commemorate the centennial celebrations of now-famous solar eclipse observations to test general relativity by Sir Arthur Eddington. We analyze the multi-epoch Spitzer observations of the expected flare between 2019 July 31 and 2019 September 6, as well as baseline observations during 2019 February-March. Observed Spitzer flux density variations during the predicted outburst time display a strong similarity with the observed optical pericenter flare from OJ 287 during 2007 September. The predicted flare appears comparable to the 2007 flare after subtracting the expected higher base-level Spitzer flux densities at 3.55 and 4.49 μm compared to the optical R-band. Comparing the 2019 and 2007 outburst lightcurves and the previously calculated predictions, we find that the Eddington flare arrived within 4 hr of the predicted time. Our Spitzer observations are well consistent with the presence of a nano-Hertz gravitational-wave emitting spinning massive binary BH that inspirals along a general relativistic eccentric orbit in OJ 287. These multi-epoch Spitzer observations provide a parametric constraint on the celebrated BH no-hair theorem.

The evolution of eclipsing binaries leads towards angular momentum and mass loss from the systems, due to stellar wind and magnetic braking. Observational investigations of low-temperature and low-mass contact binaries (or LMCBs) in the solar neighborhood provide the means for studying a large sample of such systems. The observed orbital period cut-off limit of 0.22 days is believed to be a result of evolutionary mechanisms, and such systems act as probes in investigating the very evolved stages of systems before their final coalescence. The main goal of this study is the determination of the stellar evolution tracks of these type of LMCBs, which might be correlated to the formation of blue stragglers and rapidly rotating stars.

W Ursae Majoris-type binaries belong to the old population of our Galaxy, while their metallicity is close to solar. Their physical properties, kinematics and spatial distribution reflect the properties of their stellar progenitors. This study focuses on the spatial distribution of W UMa's in our solar neighborhood within a 500 pc radius, with a combined astrometric, photometric and spectroscopic determination of their stellar parameters. The sample is carefully selected, in order to fulfill certain criteria, and has well defined metallicity and distance parameters. H-R diagram, as well as similar correlation plots (mass-radius and mass-luminosity), show that the primary (more massive) components in such systems are located close or below the ZAMS region, while secondary components seem to be evolved, as a result of their common envelope geometry. Some prominent outliers are carefully examined in order to judge the environmental properties and evolution in certain locations of the Milky Way. It is found that metallicity is not correlated with distance, but there is a weak correlation between metallicity [M/H] and evolution state, as it is expressed by the location of the systems in the H-R diagram, the type of binary (A or W), and temperature.

The existence of magnetic activity on the eclipsing binary DV Psc has been known for almost two decades. However, until recently, no evidence of periodic behaviour relevant to this activity had been found. In this study, long-term photometric observations of DV Psc are used to analyze the system's magnetic activity, seek a possible magnetic cycle and determine orbital and physical parameters. The combination of photometric and spectroscopic observations results in a unified model that describes the system over time in terms of variable spot activity. New times of minimum light are determined and an accurate astronomical ephemeris and updated O-C diagram are constructed for a total span of 19 years (1997-2017). The intense magnetic activity, as indicated by strong asymmetries in the light curves (O' Connell effect), and the periodic variation of the O-C diagram are combined to explain the system's behaviour. The existence of a third body, orbiting the eclipsing binary in an eccentric orbit, as well as a magnetic cycle are the most likely scenario.

We report the discovery of the relatively bright (V = 10.5 mag), doubly eclipsing 2+2 quadruple system CzeV1731. This is the third known system of its kind, in which the masses are determined for all four stars and both the inner and outer orbits are characterized. The inner eclipsing binaries are well-detached systems moving on circular orbits: pair A with period PA = 4.10843 d and pair B with PB = 4.67552 d. The inner binaries contain very similar components (q ≈ 1.0), making the whole system a so-called double twin. The stars in pair B have slightly larger luminosities and masses and pair A shows deeper eclipses. All four components are main-sequence stars of F/G spectral type. The mutual orbit of the two pairs around the system barycenter has a period of about 34 yr and an eccentricity of about 0.38. However, further observations are needed to reveal the overall architecture of the whole system, including the mutual inclinations of all orbits. This is a promising target for interferometry to detect the double at about 59 mas and ΔMbol < 1 mag. The RV and ETV data, and the light curves are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/642/A63

We present new photometric and spectroscopic observations and analyses for the eclipsing binary systems V1241 Tau and GQ Dra. Our photometric light and radial velocity curve analyses combined with the TESS light curves show that both are conventional semi-detached binary systems. Their absolute parameters are also derived. We present the O-C analyses of the systems and we propose the most possible orbital period modulating mechanisms. Furthermore, Fourier analyses are applied to the photometric residual data of the systems to check for the pulsational behavior of the components. We conclude that the primary component of the system GQ Dra is a δ Sct type pulsator with a dominant pulsation frequency of 18.58 d-1 based on our B filter residual light curve although it can not be justified by 30-minute cadence TESS data. No satisfactory evidence of pulsational behavior for V1241 Tau was verified. Finally, the evolutionary tracks of the components of both systems are calculated. Their locations within evolutionary diagrams are compared with other Algol-type systems

Planets In Your Hand is a science education program, that consists of a portable interactive exhibition of eight planetary surface models. The program offers the visitors a tactile experience and the opportunity to understand the diversity of the planetary surfaces in our Solar System. The planetary models have been exhibited in a series of public events since their construction in 2018 and have been visited by a wide range of audience, including visually impaired people. The project is still running, while Planets In Your Hand team is working on improvements, that foresee to a greater social and educational impact. The current work summarizes the social impact of the program through the visitors" questionnaires, comments and impressions.1. IntroductionPlanets In Your Hand (PIYH) is a science communication project, initiated at the Department of Physics of National and Kapodistrian University of Athens (NKUA). Its purpose is to reach individuals, that do not have any previous interaction with the field, trigger their interest to interactively participate in scientific activities and public events and eventually bring them closer to science and contribute in lifelong learning. This is mainly attempted through a visual and tangible representation of the planetary environments and morphologies in our Solar System (Kefala et al., 2018), that also benefits visually impaired people.The success of PIYH project is established by the Science Communication Award (EΠI2 Award 2019) in the category of "Awareness Activities and Campaigns".PIYH program is a non-formal learning experience, the importance of which is widely accepted and supported by the National Science Education Standards (National Research Council, 1996). The way science communication is organized and planned as well as the visitor"s intrinsic motivation for learning (Eshach 2007) declares this an indisputable fact.2. Data CollectionIn order to evaluate the social impact of the PIYH project, questionnaires were filled out by the visitors after their conceivable journey to our Solar System. Multiple-choice questions and a comment section were included. The goal was to examine whether the exhibition provides an overall positive experience to the visitors and to find possible ways to improve its presentation.The planetary models have been exhibited in a series of public events (Fig. 1) since the beginning of the program (Palafouta et al., 2019). This research was conducted during two major events where PIYH was presented. These are the opening of the exhibition and the Athens Science Festival 2019.Oral impressions and evaluations for every event were also made directly by the members of the PIYH team that presented the planetary surfaces. They were based on the reactions and the comments of the visitors and were really helpful. PIYH program is a non-formal learning experience, the importance of which is widely accepted and supported by the National Science Education Standards (National Research Council, 1996). The way science communication is organized and planned as well as the visitor"s intrinsic motivation for learning (Eshach 2007) declares this an indisputable fact.3. The social impactThe questionnaire provided an overall positive feedback by the visitors. As shown in Fig. 2 the majority (97.5 %) of the visitors were pleased with PIYH project and the exhibition. Early ages (≤23 years old) occupy great percentage (61 %) of the visitors, while the rest include individuals, parents or educators.The overwhelming majority of the participants (98.5%) would recommend the exhibition to a friend, enhancing the social impact of the project, while many parents and educators mentioned that this project can trigger new generations to engage with Astrophysics and science in general.More than 47% of the visitors shared their impressions through additional comments. Positive remarks were given, for the exhibition and its presentation by the PIYH team members.Visually impaired people commented positively, mentioning that exhibitions in general do not offer tangible experience, and they are limited to an oral, acoustic or pre-recorded presentation. PIYH exhibition gave them for the first time the sense of planetary scale, surface characteristics and the unique opportunity to interact with planetary surfaces.A wide range of reactions was perceived by the PIYH team members during the events with respect to the visitor"s background in astrophysics.Regardless of their age, people who were getting in touch with Planetary Science for the first time were curious to touch and feel the surface"s differences, asking questions about the internal and external structure and the atmospheric phenomena of each planet. Those who had scientific background, combined facts and knowledge from their own scientific field with the information provided during the events.On the other hand, visitors familiar with astronomy and planetary science expressed more elaborate questions, many of which concerned the current research on space exploration.4. Summary and ConclusionsThis research has shown that people of different age and scientific background can be brought together through an interactive and tangible educational experience. PIYH exhibition is considered a pleasant and memorable experience according to the conducted analysis. The project attracted individuals that have no previous interaction with the field of Planetary Science, triggering their interest and brought together people who are already interested or contribute to science. PIYH project makes this learning experience accessible to visually impaired people, encouraging this way the local community to create similar activities. It is an alternative, creative and interesting way of learning about our Solar System. Similar projects can be inspired by PIYH program and engage the general public in science through a pleasant and cognitively successful way, enhancing the communication of science in many fields.References[1] Eshach, H., 2007, Journal of Science Education and Technology, 16, 171-190.[2] Kefala, K., et al., 2018, EPSC2018,1251-2.[3] National Research Council, 1996.National Science Education Standards. Washington DC: National Academy Press.[4] Palafouta, S., et al., 2019, EPSC-DPS2019, 1816-1

The so-called "primordial family", is a recently discovered collisional family [1] that could be as old as the Solar system. It contains low-albedo asteroids and is located in the inner Main Belt. This family was identified by the V-shape technique [2] and is estimated to be at least 4 Gyr old, meaning that occurred before the giant planet instability [3]. Asteroids 2768 Gorky (1972 RX3) and 9086 (1995 SA3) are members of the primordial family and have been observed in order to determine their rotational period, spin pole and shape, which will give insights about their membership and the family evolution. The photometric data are obtained mainly by the University of Athens Observatory (UOAO) [4]. Models of the asteroids are derived by the lightcurve inversion method, which is stored in the Database of Asteroid Models from Inversion Techniques (DAMIT) [5].1. IntroductionThe so-called primordial family formed by an impact event 4 Gyr ago or even earlier, i.e. before the giant planet instability [3]. The identification of the primordial family has been done by using the V-shape technique [2]. The V-shape in (α,1/D) space is occurred due to the thermal Yarkovsky effect [6, 7, 2], where prograde-rotating asteroids drifting outwards and retrograde ones inward. As only the inward side of the V-shape of the primordial family is well defined, the members should have retrograde spin. Otherwise, the membership of asteroids with prograde spin could be disputed or the contradicting spin sense could be due to YORP cycles or a collision or a close encounter with other asteroid [8, 9]. Thus, it is essential to observe thoroughly the asteroids belonging to the primordial family, in order to determine their rotational state and shape.2. ObservationsAsteroid 2768 Gorky (1972 RX3) is member of the primordial asteroid family of the inner main belt [1]. Its rotational period is 4.507 ± 0.010 h [10], while its spin pole and shape remain unknown. Observations from Sopot Astronomy Observatory (SAO) for 2 nights in September 2019, in phase angle of 32°, are available in Asteroid Lightcurve Photometry Database (ALCDEF)[11] and present a period of 4.445 ± 0.003 h [12]. On the Observatoire de Genève website of R. Behrend [13] rotational period of the asteroid was reported to be 4.5118 ± 0.0007 h. Asteroid 2768 Gorky (1972 RX3) has been observed from UOAO for 5 nights in December 2019, in phase angles between 10° to 14°.Asteroid 9086 (1995 SA3) is member of the primordial asteroid family in the inner main belt [1]. Its rotational period, spin pole and shape remain unknown. UOAO has been observed this asteroid for 22 nights from October to November 2019, in phase angle ranging between 4° to 22°.3. Methodology and Preliminary ResultsDifferential photometry of the asteroid has been obtained for each sky field. All images were dark and flat-field corrected and unguided. The measured values are in magnitudes and have been transformed properly in relative flux. The determination of the rotational period of the two asteroids and the estimation of the spin pole and shape of 2768 Gorky (1972) has been carried out through the lightcurve inversion method stored in DAMIT [5]. 3.1 Asteroid 2768 Gorky (1972 RX3) The lightcurve of asteroid 2768 Gorky (1972 RX3) present a low amplitude of ~0.1 mag (or ~0.1 intensity units), as Figure 1 shows. Generating 50 random spin poles as initial input for DAMIT software, the results have the distribution presented in Figure 2. The plot in this figure constrains the possible spin poles of the asteroid, while the χ2 value and the dark facet percentage should have the lowest value. 3.2 Asteroid 9086 (1995 SA3) The lightcurve of asteroid 9086 (1995 SA3) present also a low amplitude of ~ 0.2 mag (or ~0.2 intensity units). The low brightness of the object in combination with the low amplitude of its lightcurve render the detection of its periodicity difficult, due to the background noise. In order to address this problem, the exposures of each night were combined in packs of 3, in order to increase the S/N and make the detection of light variation more prominent. The resulted period spectrum from DAMIT is presenting in Figure 3. 4. Summary and Conclusions The rotational period of 2768 Gorky (1972 RX3) has been calculated as 4.510 ± 0.020 h, in agreement with the literature. The spin pole is not defined well enough. It is needed observations from more than one apparition for more robust results. Asteroid 9086 (1995 SA3), despite the extensive observations, does not present a clear lightcurve, as a result to have not unique solution in the period. 5. Further work The determination of asteroids" rotational state and shape requires observations from more than one apparition. Alternatively, combing the already ground based with sparse data from sky surveys or space-borne could also drive to unique solutions. Acknowledgements We would to acknowledge Dr. Hanus J. for his valuable help and advices for the lightcurve analysis that performed. References [1] Delbo, M. et al. (2017). Science, 357, 1026-1029. [2] Bolin, B. T. et al. (2017). Icarus, 282, 290-312. [3] Tsiganis, K., et al. (2005). Nature, 435(7041), 459-461. [4] Gazeas, K. (2016). Revista Mexicana de Astronomía y Astrofísica, 48, 22-23. [5] Ďurech, J. et al. (2010). Astronomy & Astrophysics, 513, A46. (DAMIT"s website: astro.troja.mff.cuni.cz) [6] Milani, A. et al. (2014). Icarus, 239, 46-73. [7] Spoto, F. et al. (2015). Icarus, 257, 275-289. [8] Carruba, V. et al. (2013). Monthly Notices of the Royal Astronomical Society, 433(3), 2075-2096. [9] Delisle, J. B., & Laskar, J. (2012). Astronomy & Astrophysics, 540, A118. [10] Pray, D. P. et al. (2008). Minor Planet Bulletin, 35, 34-36. [11] Asteroid Lightcurve Photometry Database (ALCDEF)"s website: alcdef.org [12] Benishek, V. (2020). Minor Planet Bulletin, 47(1), 75-83. [13] Observatoire de Genève, Website of R. Behrend: obswww.unige.ch/~behrend

The discovery and analysis of the system CzeV1731 represents a rare configuration of a 2+2 quadruple star system containing two eclipsing binaries, the third system of its kind known to date. The system under investigation in this work is TYC 3929-724-1 (= 2MASS J19245582+5704084 = TIC 284482112), located at RA=19:24:55.82, DE=+57:04:08.39, with Vmax=10.5mag. We present times of eclipses and radial velocities of both pairs. (2 data files).

Context. EE Cep is one of few eclipsing binary systems with a dark, dusty disc around an invisible object similar to ɛ Aur. The system is characterised by grey and asymmetric eclipses every 5.6 yr that have significant variations in their photometric depth, ranging from ∼0.m5 to ∼2.m0. Aims: The main aim of the observational campaign of the EE Cep eclipse in 2014 was to test the model of disc precession. We expected that this eclipse would be one of the deepest with a depth of ∼2.m0. Methods: We collected multicoloured observations from almost 30 instruments located in Europe and North America. These photometric data cover 243 nights during and around the eclipse. We also analyzed low- and high-resolution spectra from several instruments. Results: The eclipse was shallow with a depth of 0.m71 in the V band. The multicoloured photometry illustrates small colour changes during the eclipse with a total amplitude of order ∼+0.m15 in the B - I colour index. We updated the linear ephemeris for this system by including new times of minima, measured from the three most recent eclipses at epochs E = 9, 10, and 11. We acquired new spectroscopic observations, covering orbital phases around the eclipse, which were not observed in the past and increased the data sample, filling some gaps and giving better insight into the evolution of the Hα and Na I spectral line profiles during the primary eclipse. Conclusions: The eclipse of EE Cep in 2014 was shallower than expected, measuring 0.m71 instead of ∼2.m0. This means that our model of disc precession needs revision. Tables A1-A29 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/639/A23

Context. Deriving physical properties of trans-Neptunian objects is important for the understanding of our Solar System. This requires observational efforts and the development of techniques suitable for these studies. Aims: Our aim is to characterize the large trans-Neptunian object (TNO) 2002 TC302. Methods: Stellar occultations offer unique opportunities to determine key physical properties of TNOs. On 28 January 2018, 2002 TC302 occulted a mv ~ 15.3 star with designation 593-005847 in the UCAC4 stellar catalog, corresponding to Gaia source 130957813463146112. Twelve positive occultation chords were obtained from Italy, France, Slovenia, and Switzerland. Also, four negative detections were obtained near the north and south limbs. This represents the best observed stellar occultation by a TNO other than Pluto in terms of the number of chords published thus far. From the 12 chords, an accurate elliptical fit to the instantaneous projection of the body can be obtained that is compatible with the near misses. Results: The resulting ellipse has major and minor axes of 543 ± 18 km and 460 ± 11 km, respectively, with a position angle of 3 ± 1 degrees for the minor axis. This information, combined with rotational light curves obtained with the 1.5 m telescope at Sierra Nevada Observatory and the 1.23 m telescope at Calar Alto observatory, allows us to derive possible three-dimensional shapes and density estimations for the body based on hydrostatic equilibrium assumptions. The effective diameter in equivalent area is around 84 km smaller than the radiometrically derived diameter using thermal data from Herschel and Spitzer Space Telescopes. This might indicate the existence of an unresolved satellite of up to ~300 km in diameter, which is required to account for all the thermal flux, although the occultation and thermal diameters are compatible within their error bars given the considerable uncertainty of the thermal results. The existence of a potential satellite also appears to be consistent with other ground-based data presented here. From the effective occultation diameter combined with absolute magnitude measurements we derive a geometric albedo of 0.147 ± 0.005, which would be somewhat smaller if 2002 TC302 has a satellite. The best occultation light curves do not show any signs of ring features or any signatures of a global atmosphere. Tables A.1-A.3 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/639/A134

Table 1 includes the data from the astrometric monitoring runs for 2002 TC302 carried out to narrow down the shadow path uncertainty, in October, November, December 2017 and January 2018 with the Sierra Nevada 1.5m telescope (Granada, Spain) and the Calar Alto 1.2m telescope (Almeria, Spain). The astrometric catalog used for the occultation prediction was Gaia DR1. Tables 2 and 3 include, respectively, all the photometry and astrometry of 2002 TC302 analyzed, from 2014 to 2019. The astrometry was derived with respect to the Gaia DR2 catalog (Cat. I/345). (3 data files).

Context. The tenuous nitrogen (N2) atmosphere on Pluto undergoes strong seasonal effects due to high obliquity and orbital eccentricity, and has recently (July 2015) been observed by the New Horizons spacecraft. Aims: The main goals of this study are (i) to construct a well calibrated record of the seasonal evolution of surface pressure on Pluto and (ii) to constrain the structure of the lower atmosphere using a central flash observed in 2015. Methods: Eleven stellar occultations by Pluto observed between 2002 and 2016 are used to retrieve atmospheric profiles (density, pressure, temperature) between altitude levels of ~5 and ~380 km (i.e. pressures from ~ 10 μbar to 10 nbar). Results: (i) Pressure has suffered a monotonic increase from 1988 to 2016, that is compared to a seasonal volatile transport model, from which tight constraints on a combination of albedo and emissivity of N2 ice are derived. (ii) A central flash observed on 2015 June 29 is consistent with New Horizons REX profiles, provided that (a) large diurnal temperature variations (not expected by current models) occur over Sputnik Planitia; and/or (b) hazes with tangential optical depth of ~0.3 are present at 4-7 km altitude levels; and/or (c) the nominal REX density values are overestimated by an implausibly large factor of ~20%; and/or (d) higher terrains block part of the flash in the Charon facing hemisphere.

We report an analysis of the first known β Cep pulsator observed by the Transiting Exoplanet Survey Satellite (TESS) mission, the runaway star PHL 346 = HN Aqr. The star, previously known as a singly periodic pulsator, has at least 34 oscillation modes excited, 12 of those in the g-mode domain and 22 p modes. Analysis of archival data implies that the amplitude and frequency of the dominant mode and the stellar radial velocity were variable over time. A binary nature would be inconsistent with the inferred ejection velocity from the Galactic disk of 420 km s-1, which is too large to be survivable by a runaway binary system. A kinematic analysis of the star results in an age constraint (23 ± 1 Myr) that can be imposed on asteroseismic modeling and that can be used to remove degeneracies in the modeling process. Our attempts to match the excitation of the observed frequency spectrum resulted in pulsation models that were too young. Hence, asteroseismic studies of runaway pulsators can become vital not only in tracing the evolutionary history of such objects, but to understand the interior structure of massive stars in general. TESS is now opening up these stars for detailed asteroseismic investigation.

We use deep Chandra and HST data to uniquely classify the X-ray binary (XRB) populations in M81 on the basis of their donor stars and local stellar populations (into early-type main sequence, yellow giant, supergiant, low-mass, and globular cluster). First, we find that more massive, redder, and denser globular clusters are more likely to be associated with XRBs. Second, we find that the high-mass XRBs (HMXBs) overall have a steeper X-ray luminosity function (XLF) than the canonical star-forming galaxy XLF, though there is some evidence of variations in the slopes of the sub-populations. On the other hand, the XLF of the prototypical starburst M82 is described by the canonical powerlaw (αcum ∼ 0.6) down to LX ∼ 1036 erg s-1. We attribute variations in XLF slopes to different mass transfer modes (Roche-lobe overflow versus wind-fed systems).

The eclipsing binary system DV Psc has been known to be magnetically active for almost two decades. However, there has been no evidence of a magnetic cycle on this system until recently. This study focuses on the long-term photometric monitoring of DV Psc between 2005-2017. A total of 50 individual light curves in BVRI optical bands were collected, in order to investigate its magnetic activity and cycle, as well as determine its orbital and physical properties. The combined photometric and spectroscopic observations of this study resulted in a unified model for the system, which describes accurately the light curves throughout the years, as a result of the variable spot activity. A total of 105 new times of minimum light are calculated through the entire time span of observations and they are combined with the 203 bibliographic ones since 1997, increasing significantly the existing sample. This resulted in an accurate ephemeris and an updated O-C diagram for a total span of 20 yr (1997-2017). It is found that the system exhibits intense magnetic activity, which is shown through the strong asymmetries on the light curves (O'Connell effect) and the periodic variation of the O-C diagram. The existence of a third body, orbiting the eclipsing binary with a period of Porb=9.79±0.60 yr in an eccentric orbit with eccentricity e=0.83±0.24, as well as a magnetic cycle of 14.74±0.84 yr are most likely connected with this variability. The absolute physical parameters of the system are calculated for a new and unified model, which explains the light curves through the entire observing season. The evolution state of DV Psc is studied through the mass-radius and temperature-luminosity (HR) diagrams, as well as the location of the two components with respect to the ZAMS and TAMS region.

The Planets In Your Hand project was granted by Europlanet in the frame of Public Engagement Funding Scheme in 2017. It consists of a portable interactive exhibition of planetary surface models, embedded in square frames. The project offers the visitors a chance to see, learn and understand the diversity of the planetary surfaces in our Solar System, including the different conditions such as temperature, wind and atmosphere. Since the beginning of the project, the planetary surfaces have been presented to a wide range of audience, including visually impaired people, preliminary school and high school students as well as university researchers.

We obtained high-resolution spectra of HD 221416 using several facilities within the TESS Follow-up Observation Program (TFOP), including HIRES (Vogt et al. 1994SPIE.2198..362V) on the 10 m telescope at Keck Observatory (Maunakea, Hawai'i); the Hertzsprung SONG Telescope at Teide Observatory (Tenerife; Grundahl et al. 2017ApJ...836..142G); HARPS (Mayor et al. 2003Msngr.114...20M), FEROS (Kaufer et al. 1999Msngr..95....8K), Coralie (Queloz et al. 2001Msngr.105....1Q), and FIDEOS (Vanzi et al. 2018MNRAS.477.5041V) on the MPG/ESO 3.6 m, 2.2 m, 1.2 m, and 1 m telescopes at La Silla Observatory (Chile); Veloce (Gilbert et al. 2018SPIE10702E..0YG) on the 3.9 m Anglo-Australian Telescope at Siding Spring Observatory (Australia); TRES (Furesz 2008, PhD thesis Univ. Szeged) on the 1.5 m Tillinghast reflector at the F. L. Whipple Observatory (Mt. Hopkins, Arizona); and iSHELL (Rayner et al. 2012SPIE.8446E..2CR) on the NASA IRTF Telescope (Maunakea, Hawai'i). All spectra used in this paper were obtained between 2018 November 11 and December 30 and have a minimum spectral resolution of R~44000. (1 data file).

We present the discovery of HD 221416 b, the first transiting planet identified by the Transiting Exoplanet Survey Satellite (TESS) for which asteroseismology of the host star is possible. HD 221416 b (HIP 116158, TOI-197) is a bright (V = 8.2 mag), spectroscopically classified subgiant that oscillates with an average frequency of about 430 μHz and displays a clear signature of mixed modes. The oscillation amplitude confirms that the redder TESS bandpass compared to Kepler has a small effect on the oscillations, supporting the expected yield of thousands of solar-like oscillators with TESS 2 minute cadence observations. Asteroseismic modeling yields a robust determination of the host star radius (R ⋆ = 2.943 ± 0.064 R ⊙), mass (M ⋆ = 1.212 ± 0.074 M ⊙), and age (4.9 ± 1.1 Gyr), and demonstrates that it has just started ascending the red-giant branch. Combining asteroseismology with transit modeling and radial-velocity observations, we show that the planet is a “hot Saturn” (R p = 9.17 ± 0.33 R ⊕) with an orbital period of ∼14.3 days, irradiance of F = 343 ± 24 F ⊕, and moderate mass (M p = 60.5 ± 5.7 M ⊕) and density (ρ p = 0.431 ± 0.062 g cm-3). The properties of HD 221416 b show that the host-star metallicity-planet mass correlation found in sub-Saturns (4-8 R ⊕) does not extend to larger radii, indicating that planets in the transition between sub-Saturns and Jupiters follow a relatively narrow range of densities. With a density measured to ∼15%, HD 221416 b is one of the best characterized Saturn-size planets to date, augmenting the small number of known transiting planets around evolved stars and demonstrating the power of TESS to characterize exoplanets and their host stars using asteroseismology.

Systematic monitoring of specific targets in the optical regime was historically applied on a very narrow sample of known variable stars. The discovery of blazars in the 20th century brought to the foreground the need for new global sky surveys, covering the entire sky and fainter sources. Full-sky surveys are conducted more easily from space observatories, while radio telescopes perform follow up observations from the ground. Blazars are detected in a wide range of energies, while they exhibit strong variability in various wavelengths from γ-rays and X-rays to the optical and radio domain. This results in a detailed classification, according to their emission properties in each region. The rapid variability in optical domain makes blazars interesting targets for optical sky surveys, offering a new opportunity to study their variability in the time domain. Digital sky surveys in optical and near-IR found a fertile ground with the aid of sensitive sensors. Only a few dedicated programs are focusing on blazar variability, a trend which evolved rapidly in the last decade. Modern techniques, in combination with dedicated sky survey programs lead towards a new era of long-term monitoring of blazars, aiming towards the search or variability on various time scales. In this work, an overview of blazar optical surveys and monitoring projects is given, addressing the major points of each one, and highlighting the constraints that the long-term study of blazars will bring through future international campaigns.

Results from regular monitoring of relativistic compact binaries like PSR 1913+16 are consistent with the dominant (quadrupole) order emission of gravitational waves (GWs). We show that observations associated with the binary black hole (BBH) central engine of blazar OJ 287 demand the inclusion of gravitational radiation reaction effects beyond the quadrupolar order. It turns out that even the effects of certain hereditary contributions to GW emission are required to predict impact flare timings of OJ 287. We develop an approach that incorporates this effect into the BBH model for OJ 287. This allows us to demonstrate an excellent agreement between the observed impact flare timings and those predicted from ten orbital cycles of the BBH central engine model. The deduced rate of orbital period decay is nine orders of magnitude higher than the observed rate in PSR 1913+16, demonstrating again the relativistic nature of OJ 287's central engine. Finally, we argue that precise timing of the predicted 2019 impact flare should allow a test of the celebrated black hole “no-hair theorem” at the 10% level.

Blazar OJ287 exhibits large thermal flares at least twice every 12 years. The times of these flares have been predicted successfully using the model of a quasi-Keplerian eccentric black hole binary where the secondary impacts the accretion disk of the primary, creating the thermal flares. New measurements of the historical light curve have been combined with the observations of the 2015 November/December flare to identify the impact record since year 1886, and to constrain the orbit of the binary. The orbit solution shows that the binary period, now 12.062 year, is decreasing at the rate of 36 days per century. This corresponds to an energy loss to gravitational waves that is 6.5 +- 4 % less than the rate predicted by the standard quadrupolar gravitational wave (GW) emission. We show that the difference is due to higher order gravitational radiation reaction terms that include the dominant order tail contributions.

Blazar OJ287 exhibits large thermal flares at least twice every 12 years. The times of these flares have been predicted successfully using the model of a quasi-Keplerian eccentric black hole binary where the secondary impacts the accretion disk of the primary, creating the thermal flares. New measurements of the historical light curve have been combined with the observations of the 2015 November/December flare to identify the impact record since year 1886, and to constrain the orbit of the binary. The orbital solution shows that the binary period, now 12.062 years, is decreasing at the rate of 36 days per century. This corresponds to an energy loss to gravitational waves that is 6.5 +/- 4 % less than the rate predicted by the standard quadrupolar gravitational wave (GW) emission. We show that the difference is due to higher order gravitational radiation reaction terms that include the dominant order tail contributions.

Mankind has an inherent desire to explore the planets, which is closely connected with the nature and curios- ity of human beings. Unfortunately, the actual explo- ration is a privilege of only a few people, at least in the near future. The presented project, entitled "Planets In Your Hand" (PIYH) gives everyone the chance to see, touch and feel the differences of each planetary sur- face in our Solar System and learn about the world we all live in. The PIYH project consists of a portable ex- hibition of planetary surface models in square frames. It is carefully designed and addressed to both young and elder people, families, students, educational insti- tutions and especially to visually impaired individuals, giving them a unique opportunity to meet and get fa- miliar with planetary science.

We present the results of our power spectral density analysis for the BL Lac object OJ 287, utilizing the Fermi-LAT survey at high-energy γ-rays, Swift-XRT in X-rays, several ground-based telescopes and the Kepler satellite in the optical, and radio telescopes at GHz frequencies. The light curves are modeled in terms of continuous-time autoregressive moving average (CARMA) processes. Owing to the inclusion of the Kepler data, we were able to construct for the first time the optical variability power spectrum of a blazar without any gaps across ∼6 dex in temporal frequencies. Our analysis reveals that the radio power spectra are of a colored-noise type on timescales ranging from tens of years down to months, with no evidence for breaks or other spectral features. The overall optical power spectrum is also consistent with a colored noise on the variability timescales ranging from 117 years down to hours, with no hints of any quasi-periodic oscillations. The X-ray power spectrum resembles the radio and optical power spectra on the analogous timescales ranging from tens of years down to months. Finally, the γ-ray power spectrum is noticeably different from the radio, optical, and X-ray power spectra of the source: we have detected a characteristic relaxation timescale in the Fermi-LAT data, corresponding to ∼150 days, such that on timescales longer than this, the power spectrum is consistent with uncorrelated (white) noise, while on shorter variability timescales there is correlated (colored) noise.

During the 2014-2015 mutual events season, the Institut de Mécanique Céleste et de Calcul des Éphémérides (IMCCE), Paris, France, and the Sternberg Astronomical Institute (SAI), Moscow, Russia, led an international observation campaign to record ground-based photometric observations of Galilean moon mutual occultations and eclipses. We focused on processing the complete photometric observations data base to compute new accurate astrometric positions. We used our method to derive astrometric positions from the light curves of the events. We developed an accurate photometric model of mutual occultations and eclipses, while correcting for the satellite albedos, Hapke's light scattering law, the phase effect, and the limb darkening. We processed 609 light curves, and we compared the observed positions of the satellites with the theoretical positions from IMCCE NOE-5-2010-GAL satellite ephemerides and INPOP13c planetary ephemeris. The standard deviation after fitting the light curve in equatorial positions is ±24 mas, or 75 km at Jupiter. The rms (O-C) in equatorial positions is ±50 mas, or 150 km at Jupiter.

We present 114 times of minima of 6 ultra-short orbital period eclipsing binaries.

The X-ray source 1RXS J051439.2-021615 (RA 05h14m39.20s, Dec -02d16m15.00, J2000.0) showed an optical V-band flare, during the night of 25/26 January 2018 (JD=2458114).

We report optical photometric and polarimetric observations of the blazar OJ 287 gathered during 2016/17. The high level of activity, noticed after the General Relativity Centenary flare, is argued to be part of the follow-up flares that exhibited high levels of polarization and originated in the primary black hole jet. We propose that the follow-up flares were induced as a result of accretion disk perturbations, travelling from the site of impact towards the primary SMBH. The timings inferred from our observations allowed us to estimate the propagation speed of these perturbations. Additionally, we make predictions for the future brightness of OJ 287.

We present a detailed analysis of the X-ray point source population of M81. By identifying HST counterparts to Chandra sources, taking into account the chance coincidence probability, we classify a large fraction of the X-ray point source population with unique counterparts: high-mass X-ray binaries and low-mass X-ray binaries in globular clusters. We then compare the shapes of the uncontaminated X-ray luminosity functions and the X-ray properties of sources of different classes to models and other existing work. We also calculate scaling relations with the star formation rate and stellar mass between the different classes of sources in global and sub-galactic scales. One initial, primary result is that the more massive and dense globular clusters are more likely to be associated with X-ray binaries.

Haumea—one of the four known trans-Neptunian dwarf planets—is a very elongated and rapidly rotating body. In contrast to other dwarf planets, its size, shape, albedo and density are not well constrained. The Centaur Chariklo was the first body other than a giant planet known to have a ring system, and the Centaur Chiron was later found to possess something similar to Chariklo’s rings. Here we report observations from multiple Earth-based observatories of Haumea passing in front of a distant star (a multi-chord stellar occultation). Secondary events observed around the main body of Haumea are consistent with the presence of a ring with an opacity of 0.5, width of 70 kilometres and radius of about 2,287 kilometres. The ring is coplanar with both Haumea’s equator and the orbit of its satellite Hi’iaka. The radius of the ring places it close to the 3:1 mean-motion resonance with Haumea’s spin period—that is, Haumea rotates three times on its axis in the time that a ring particle completes one revolution. The occultation by the main body provides an instantaneous elliptical projected shape with axes of about 1,704 kilometres and 1,138 kilometres. Combined with rotational light curves, the occultation constrains the three-dimensional orientation of Haumea and its triaxial shape, which is inconsistent with a homogeneous body in hydrostatic equilibrium. Haumea’s largest axis is at least 2,322 kilometres, larger than previously thought, implying an upper limit for its density of 1,885 kilograms per cubic metre and a geometric albedo of 0.51, both smaller than previous estimates. In addition, this estimate of the density of Haumea is closer to that of Pluto than are previous estimates, in line with expectations. No global nitrogen- or methane-dominated atmosphere was detected.

We present 110 times of minima of 7 ultra-short orbital period eclipsing binaries.

We present 120 times of minima of 8 eclipsing binaries.

The occultation of the 14th mag star UCAC4 345-180315 by Pluto on the evening of the 19th of July, 2016 could be observed from large parts of Europe, middle east and northern Africa as well. A campaign had been organized with for many observers and observatories throughout Europe and other countries. Professional as well as amateur observatories and observers shared in a PRO-AM cooperation to achieve the highest possible degree of coverage. The scientific goal was the ongoing monitoring of Pluto's atmosphere, waiting for a possible shrinking of its pressure due to the increasing distance of Pluto from the sun.

We carried out a pilot campaign of radio and optical band intra-day variability (IDV) observations of five blazars (3C66A, S5 0716+714, OJ287, B0925+504 and BL Lacertae) on 2015 December 18-21 by using the radio telescope in Effelsberg (Germany) and several optical telescopes in Asia, Europe and America. After calibration, the light curves from both 5 GHz radio band and the optical R band were obtained, although the data were not smoothly sampled over the sampling period of about four days. We tentatively analyse the amplitudes and time-scales of the variabilities, and any possible periodicity. The blazars vary significantly in the radio (except 3C66A and BL Lacertae with only marginal variations) and optical bands on intra- and inter-day time-scales, and the source B0925+504 exhibits a strong quasi-periodic radio variability. No significant correlation between the radio- and optical-band variability appears in the five sources, which we attribute to the radio IDV being dominated by interstellar scintillation whereas the optical variability comes from the source itself. However, the radio- and optical-band variations appear to be weakly correlated in some sources and should be investigated based on well-sampled data from future observations.

The presence of a supermassive binary black hole (BBH) central engine in OJ287 is revealed by impact flares which arise from the secondary impacting the accretion disk of the primary. These flares have been observed since 1913 from the study of old photographic plates and from recent observational campaigns, 9 events in total. In addition, the secondary induces flares by affecting the accretion rate of the primary. The records for the latter events start in 1900, and they have been observed for each of the 10 cycles since then. Because of orbital precession, the impact flares times do not follow any simple rule. However, since the BBH - accretion disk impact model was proposed in 1995, the optical flux behavior of OJ287 has become highly predictable. The latest predictions were given for the late 2015 - early 2017 season. These have now been verified. The impact flare started on 2015 November 25, during the Centenary of Einstein’s General Relativity, followed by induced accretion flares which peaked on 2016 March and 2016 October. The nine impact flares specify the two parameters of the standard accretion disk and the 6 parameters of the BBH orbit uniquely. The main remaining uncertainty has to do with the exact way the gravitational wave (GW) emission affects the orbit. The impact flare observations demand that the GW emission is a combination of the instantaneous and hereditary effects appearing at the Post Newtonian orders 2.5PN, 3.5PN and 4PN, and all of them have to be incorporated while modeling the dynamics of the central engine BBH in OJ287. Previously, only the standard 2.5PN “Newtonian” GW terms have been used. We develop a simplified way of incorporating the higher order General Relativistic effects and obtain revised estimates for various BBH parameters. The improved BBH dynamics makes specific predictions for the occurrences of the expected impact flares in future and thereby provide additional strong field tests of General Relativity.

We present 106 times of minima of 27 eclipsing binaries.

On April 23, 2014, the Swift satellite responded to a hard X-ray transient detected by its Burst Alert Telescope, which turned out to be a stellar flare from a nearby, young M dwarf binary DG~CVn. Observations at X-ray, UV and optical wavelengths of the main impulsive flare and subsequent smaller events reveal a complex pattern of flare events extending over about three weeks. We find that the X-ray spectrum of the primary outburst can be adequately described by either a single very high temperature plasma or a nonthermal thick-target bremmstrahlung model. By evaluating accompanying data of this event and analysis of a second brightening, we argue that the thermal interpretation is more likely on energetic grounds. The primary outburst lasted a few hours and produced the highest temperature thermal plasmas ever seen spectroscopically over the 0.3-100 keV range in a stellar flare, at TX of 300 MK. The X-ray luminosity of the main flare exceeded the bolometric luminosity of the brighter component (LX >1.6Lbol) for ~360 seconds. The first event was followed by a comparably energetic event almost a day later, whose coverage at X-ray and optical wavelengths enables inferences about it and the first event. In particular we find evidence for stellar radius-sized coronal loops filled with dense (ne>1012 cm-3) coronal plasma. The radiated energy in X-rays and white light reveal these first two events to be some of the most energetic X-ray and white light flares from an M dwarf. These structures require large coronal magnetic field strengths (a few kG for the first event, hundreds of Gauss for the second) to confine the plasma, and we thus predict an extremely high photospheric magnetic field strength of several kiloGauss.

On 2014 April 23, the Swift satellite responded to a hard X-ray transient detected by its Burst Alert Telescope, which turned out to be a stellar flare from a nearby, young M dwarf binary DG CVn. We utilize observations at X-ray, UV, optical, and radio wavelengths to infer the properties of two large flares. The X-ray spectrum of the primary outburst can be described over the 0.3-100 keV bandpass by either a single very high-temperature plasma or a nonthermal thick-target bremsstrahlung model, and we rule out the nonthermal model based on energetic grounds. The temperatures were the highest seen spectroscopically in a stellar flare, at T X of 290 MK. The first event was followed by a comparably energetic event almost a day later. We constrain the photospheric area involved in each of the two flares to be >1020 cm2, and find evidence from flux ratios in the second event of contributions to the white light flare emission in addition to the usual hot, T ∼ 104 K blackbody emission seen in the impulsive phase of flares. The radiated energy in X-rays and white light reveal these events to be the two most energetic X-ray flares observed from an M dwarf, with X-ray radiated energies in the 0.3-10 keV bandpass of 4 × 1035 and 9 × 1035 erg, and optical flare energies at E V of 2.8 × 1034 and 5.2 × 1034 erg, respectively. The results presented here should be integrated into updated modeling of the astrophysical impact of large stellar flares on close-in exoplanetary atmospheres.

The detection of periodicity in the broadband non-thermal emission of blazars has so far been proven to be elusive. However, there are a number of scenarios that could lead to quasi-periodic variations in blazar light curves. For example, an orbital or thermal/viscous period of accreting matter around central supermassive black holes could, in principle, be imprinted in the multi-wavelength emission of small-scale blazar jets, carrying such crucial information about plasma conditions within the jet launching regions. In this paper, we present the results of our time series analysis of the ∼9.2 yr long, and exceptionally well-sampled, optical light curve of the BL Lac object OJ 287. The study primarily used the data from our own observations performed at the Mt. Suhora and Kraków Observatories in Poland, and at the Athens Observatory in Greece. Additionally, SMARTS observations were used to fill some of the gaps in the data. The Lomb-Scargle periodogram and the weighted wavelet Z-transform methods were employed to search for possible quasi-periodic oscillations in the resulting optical light curve of the source. Both methods consistently yielded a possible quasi-periodic signal around the periods of ∼400 and ∼800 days, the former with a significance (over the underlying colored noise) of ≥slant 99 % . A number of likely explanations for this are discussed, with preference given to a modulation of the jet production efficiency by highly magnetized accretion disks. This supports previous findings and the interpretation reported recently in the literature for OJ 287 and other blazar sources.

We analyse the light curve in the R band of the blazar OJ287, gathered during the 2015/2016 observing season. We did a search for quasi-periodic oscillations (QPOs) using several methods over a wide range of timescales. No statistically significant periods were found in the high-frequency domain both in the ground-based data and in Kepler observations. In the longer-period domain, the Lomb-Scargle periodogram revealed several peaks above the 99% significance level. The longest one—about 95 days—corresponds to the innermost stable circular orbit (ISCO) period of the more massive black hole. The 43-day period could be an alias, or it can be attributed to accretion in the form of a two-armed spiral wave.

In this paper, we present a combined photometric, spectroscopic, and orbital period study of three early-type eclipsing binary systems: XZ Aql, UX Her, and AT Peg. As a result, we have derived the absolute parameters of their components and, on that basis, we discuss their evolutionary states. Furthermore, we compare their parameters with those of other binary systems and with theoretical models. An analysis of all available up-to-date times of minima indicated that all three systems studied here show cyclic orbital changes; their origin is discussed in detail. Finally, we performed a frequency analysis for possible pulsational behavior, and as a result we suggest that XZ Aql hosts a δ Scuti component.

OJ287 goes through large optical flares twice each 12 years. The times of these flares have been predicted successfully now 5 times using a black hole binary model. In this model a secondary black hole goes around a primary black hole, impacting the accretion disk of the latter twice per orbital period, creating a thermal flare. Together with 6 flares from the historical data base, the set of flare timings determines uniquely the 7 parameters of the model: the two masses, the primary spin, the major axis, eccentricity and the phase of the orbit, plus a time delay parameter that gives the extent of time between accretion disk impacts and the related optical flares. Based on observations by the OJ287-15/16 Collaboration, OJ287 went into the phase of rapid flux rise on November 25, on the centenary of Einstein’s General Relativity, and peaked on December 5. At that time OJ287 was the brightest in over 30 years in optical wavelengths. The flare was of low polarization, and did not extend beyond the optical/UV region of the spectrum. On top of the main flare there were a number of small flares; their excess brightness correlates well with the simultaneous X-ray data. With these properties the main flare qualifies as the marker of the orbit of the secondary going around the primary black hole. Since the orbit solution is strongly over-determined, its parameters are known very accurately, at better than one percent level for the masses and the spin. The next flare is predicted to peak on July 28, 2019.Detailed monitoring of this event should allow us to test, for the first time, the celebrated black hole no-hair theorem for a massive black hole at the 10% level. The present data is consistent with the theorem only at a 30% level. The main difficulty in observing OJ287 from Earth at our predicted epoch is its closeness to the sun. Therefore, it is desirable to monitor OJ287 from a space-based telescope not in the vicinity of Earth. Unfortunately, this unique opportunity for testing the above celebrated theorem of General Relativity using OJ287 will not be available again until after several orbital cycles.The full list of participants in the OJ287-15/16 Collaboration is found in ApJL 819, L37, 2016.

We present 107 times of minima of 19 eclipsing binaries.

We report the discovery of the new multi-periodic variable star GSC 0476-1362. Analysis of the observed light curves shows a complex variability pattern, as a result of a multi-periodic behaviour.

OJ 287 is a quasi-periodic quasar with roughly 12 year optical cycles. It displays prominent outbursts that are predictable in a binary black hole model. The model predicted a major optical outburst in 2015 December. We found that the outburst did occur within the expected time range, peaking on 2015 December 5 at magnitude 12.9 in the optical R-band. Based on Swift/XRT satellite measurements and optical polarization data, we find that it included a major thermal component. Its timing provides an accurate estimate for the spin of the primary black hole, χ =0.313+/- 0.01. The present outburst also confirms the established general relativistic properties of the system such as the loss of orbital energy to gravitational radiation at the 2% accuracy level, and it opens up the possibility of testing the black hole no-hair theorem with 10% accuracy during the present decade.

We have continued monitoring the blazar OJ287 in the optical at several sites after its November/December, 2015 outburst. After a further large outburst that started in the beginning of February, 2016 (Atels #8667, #8697, #8705) OJ287 remained brighter than 14th magnitude [in R] for the rest of the month.

OJ287 has been monitored in the optical wavelength with small telescopes since the beginning of September, 2015. In November and December, an unprecedented outburst in the optical band was noticed (ATels: 8382, 8395, 8401, 8411, 8438), with OJ287 reaching 12.9 mag in the R filter.

Only a few red dwarf flaring stars in the solar neighbourhood have undergone exceptional events called superflares. They have been detected with high-energy satellites (Swift) and have been proven to be powerful events (both in intensity and energy) and potentially hazardous for any extraterrestial life. The physics of these events can be understood as an extrapolation of the (much) weaker activity already occurring in the most powerful solar flares occurring in the Sun. Nevertheless, the origin (why?) of these superflares occur is currently unknown. A recent study presents the optical and X-ray long-term evolution of the emission by the super-flare from the red-dwarf star DG CVn undertaken in 2 014. In that paper we comment on the context of these observations and on the properties that can be derived through the analysis of them.

A fully automatic remote telescope and dome control system has been installed at the University of Athens Observatory (UOAO) in August 2012. It was constructed in the Laboratory of Astronomy and Applied Optics of the department and incorporated the already existing automation for observations and data gathering techniques. The system proved to be reliable and functions faultlessly up to date, enabling the astronomers to observe remotely from any place, using the network. The observing nights have been increased significantly after the first year of remote operation, reaching the number of 280 observing nights per year (77% annual usage), half of which are characterized as photometric nights of highest quality. This utility favours long-term monitoring projects of blazars and long periodic variables in general.

A model that contains a massive BH binary was proposed to explain the double peaked quasi-periodical (roughly about 12 years) outbursts of the blazar OJ287.

We present 110 times of minima of 23 eclipsing binaries.

We present the results of a spectroscopic, photometric and orbital period variation analysis of the detached eclipsing binary V482 Per. We derived the absolute parameters of the system (M1 = 1.51 M⊙, M2 = 1.29 M⊙, R1 = 2.39 R⊙, R2 = 1.45 R⊙, L1 = 10.15 L⊙, L2 = 3.01 L⊙) for the first time in literature, based on an analysis of our own photometric and spectroscopic observations. We confirm the nature of the variations observed in the system's orbital period, suggested to be periodic by earlier works. A light time effect due to a physically bound, star-sized companion (M3 = 2.14 M⊙) on a highly eccentric (e = 0.83) orbit, seems to be the most likely cause. We argue that the companion can not be a single star but another binary instead. We calculated the evolutionary states of the system's components, and we found that the primary is slightly evolving after the main sequence, while the less massive secondary lies well inside it.

We present results from the application of Kopal's method for eclipsing binaries to the analysis of the transit light curve of exoplanet HD 209458, using very accurate photometric observations made with the Hubble Space Telescope. The fitting of the model parameters to the data was achieved by use of the Simulated Annealing algorithm. Furthermore, we utilized a transformation of the limb darkening coefficients that negates the need to specify limits for their values (uninformative sampling), yet still yield physically plausible results.

Contact binaries are the most frequently observed type of eclipsing star system. They are small, cool, low-mass binaries belonging to a relatively old stellar population. They follow certain empirical relationships that closely connect a number of physical parameters with each other, largely because of constraints coming from the Roche geometry. As a result, contact binaries provide an excellent test of stellar evolution, specifically for stellar merger scenarios. Observing campaigns by many authors have led to the cataloging of thousands of contact binaries and enabled statistical studies of many of their properties. A large number of contact binaries have been found to exhibit extraordinary behavior, requiring follow-up observations to study their peculiarities in detail. For example, a doubly-eclipsing quadruple system consisting of a contact binary and a detached binary is a highly constrained system offering an excellent laboratory to test evolutionary theories for binaries. A new observing project was initiated at the University of Athens in 2012 in order to investigate the possible lower limit for the orbital period of binary systems before coalescence, prior to merging.

In the course of a photometric campaign undertaken at the University of Athens Observatory, the target 1SWASP J093010.78+533859.5 was observed and found to be a doubly-eclipsing quadruple system. This system was known as a contact binary since 2013, with an orbital period of 0.2277 days, and a detached binary lies along the same direction. Hipparcos astrometric observations confirm this finding, showing two stars with a common proper motion and an angular separation of only 1.7 arcsec. This favors the scenario of a true quadruple system, making 1SWASP J093010.78+533859.5 one of the very rare doubly-eclipsing quadruple systems known to date.

We present 105 times of minima of 33 eclipsing binaries.

We present new photometric observations of two eclipsing binary systems, V1241 Tau and GQ Dra. We use the following methodology: initially, the Wilson-Devinney code is applied to the light curves in order to determine the photometric elements of the systems. Then, the residuals are analysed using Fourier techniques. The results are the following. One frequency can be possibly attributed to a real light variation of V1241 Tau, while there is no evidence of pulsations in the light curve of GQ Dra.

We present the recent photometric monitoring of blazar Mrk421, obtained from the Gerostathopouleio Observatoty at University of Athens. Follow-up observations have been performed on this source after a highly energetic flare which occurred on 13 April, 2013. The flare was observed in X- rays by Nustar & Swift and in GeV - TeV gamma-rays by the Fermi satellite and MAGIC/VERITAS telescopes respectively. Continuous photometric monitoring in the optical BVRI bands during 3 months after the flaring activity reveals a quasi-periodic light variation. This is one of the few times that Mrk 421 was observed for such a long period without large observational gaps. We perform Fourier analysis of the almost uninterrupted 3-months-long dataset in order to get insight on the characteristic timescales of the system. We discuss also possible origins of the optical variability by performing cross-correlation analysis of the optical and of the simultaneous X-ray emission detected with XRT onboard the SWIFT orbital satellite.

Selected CCD photometry of the nova forwarded by E. Waagen, AAVSO: Aug. 13.955 UT, [10.2 (G. Murawski, Suwalki, Poland; V-band); 14.817, V = 6.5 (I. Sergey, Molodechno, Belarus); 14.872, V = 6.43 (P. Hallsten, Vasterhaninge, Sweden); 14.876, B = 6.43 (L. Corp, Rodez, France); 14.877, V = 6.11 (Corp); 14.878, R = 5.68 (Corp); 15.837, B = 5.35 (T. Wikander, Borlange, Sweden); 15.883, V = 5.13 (A. Sonka, Bucharest, Romania); 16.037, I = 4.60 (K. Gazeas, Athens, Greece); 16.035, R = 4.71 (Gazeas); 16.033, V = 4.80 (Gazeas); 16.031, B = 4.97 (Gazeas); 16.279, V = 4.44 (A. Oksanen, Muurame, Finland); 16.278, B = 4.82 (Oksanen);

Our recent ground-based BVRI CCD observations of the eclipsing binary system EL Aqr provide the most accurate photometric light variation obtained so far for this system. In addition to the photometric observations, a revision of all existing spectroscopic data is carried out and a new mass ratio is derived. The combined photometric and spectroscopic data are analyzed using the Wilson- Devinney light curve synthesis code and new geometric and photometric elements are calculated. EL Aqr is found to be in contact configuration, showing partial eclipses, and one of the smallest mass ratios ever found on contact binaries.

New variable stars were discovered at the University of Athens Observatory, during data reduction in the frame of BVRI CCD observations of the eclipsing variable stars between August 2012 and June 2013. The photometric data were used in order to classify the new variables and calculate their principle photometric properties, such as their orbital period and amplitude of variation. All new discoveries are classified as contact eclipsing binaries of W UMa-type. New times of minimum light are determined and the astronomical ephemeris is calculated for each individual system.

We present a catalog of photometric and spectroscopic data collected during and around of EE Cep eclipses in 1997, 2003 and 2008/9. The photometric data were obtained with 25 different instruments, mainly in Johnson-Cousins UBV(RI)C bands, and some small sets of the data were obtained with narrow interference filters, placed on, or around of H-alpha and H-beta lines. The photometry from the last two eclipses have been compiled into the set of consistent photometric data in UBV(RI)C bands, that has been used in the paper to modelling of the eclipses in EE Cep system. The 100 of spectra released in this catalog were obtained with 8 of different spectrographs at low, medium and high resolutions (R in range ~2000-48000). About half of them (54 spectra) cover narrow ranges of order 200 Angstroms with H-alpha, H-beta lines or NaI dublet, and the second part (46 spectra) cover longer ranges of wavelengths - several thousands of Angstroms in the visual domain or in the near infrared. (7 data files).

Context. EE Cep is an unusual long-period (5.6 yr) eclipsing binary discovered during the mid-twentieth century. It undergoes almost-grey eclipses that vary in terms of both depth and duration at different epochs. The system consists of a Be type star and a dark dusty disk around an invisible companion. EE Cep together with the widely studied ɛ Aur are the only two known cases of long-period eclipsing binaries with a dark, dusty disk component responsible for periodic obscurations. Aims: Two observational campaigns were carried out during the eclipses of EE Cep in 2003 and 2008/9 to verify whether the eclipsing body in the system is indeed a dark disk and to understand the observed changes in the depths and durations of the eclipses. Methods: Multicolour photometric data and spectroscopic observations performed at both low and high resolutions were collected with several dozen instruments located in Europe and North America. We numerically modelled the variations in brightness and colour during the eclipses. We tested models with different disk structure, taking into consideration the inhomogeneous surface brightness of the Be star. We considered the possibility of disk precession. Results: The complete set of observational data collected during the last three eclipses are made available to the astronomical community. The 2003 and 2008/9 eclipses of EE Cep were very shallow. The latter is the shallowest among all observed. The very high quality photometric data illustrate in detail the colour evolution during the eclipses for the first time. Two blue maxima in the colour indices were detected during these two eclipses, one before and one after the photometric minimum. The first (stronger) blue maximum is simultaneous with a "bump" that is very clear in all the UBV(RI)C light curves. A temporary increase in the I-band brightness at the orbital phase ~0.2 was observed after each of the last three eclipses. Variations in the spectral line profiles seem to be recurrent during each cycle. The Na i lines always show at least three absorption components during the eclipse minimum and strong absorption is superimposed on the Hα emission. Conclusions: These observations confirm that the eclipsing object in EE Cep system is indeed a dark, dusty disk around a low luminosity object. The primary appears to be a rapidly rotating Be star that is strongly darkened at the equator and brightened at the poles. Some of the conclusions of this work require verification in future studies: (i) a complex, possibly multi-ring structure of the disk in EE Cep; (ii) our explanation of the "bump" observed during the last two eclipses in terms of the different times of obscuration of the hot polar regions of the Be star by the disk; and (iii) our suggested period of the disk precession (~11-12 Porb) and predicted depth of about 2 m for the forthcoming eclipse in 2014. Appendix A is available in electronic form at http://www.aanda.orgTables B.1-B.36 are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/544/A53

VFTS 682, a very massive and very hot Wolf-Rayet (WR) star recently discovered in the Large Magellanic Cloud near the famous star cluster R136, might be providing us with a glimpse of a missing link in our understanding of Long Gamma-Ray Bursts (LGRBs), including dark GRBs. It is likely its properties result from chemically homogeneous evolution (CHE), believed to be a key process for a massive star to become a GRB. It is also heavily obscured by dust extinction, which could make it a dark GRB upon explosion. Using Spitzer data we investigate the properties of interstellar dust in the vicinity of R136, and argue that its high obscuration is not unusual for its environment and that it could indeed be a slow runaway (``walkaway'') from R136. Unfortunately, based on its current mass loss rate, VFTS 682 is unlikely to become a GRB, because it will lose too much angular momentum at its death. If it were to become a GRB, it probably would also not be dark, either escaping or destroying its surrounding dusty region. Nevertheless, it is a very interesting star, deserving further studies, and being one of only three presently identified WR stars (two others in the Small Magellanic Cloud) that seems to be undergoing CHE.

We discuss the origin and evolution of low mass contact binaries with Porb shorter than 0.3 d that have properties somewhat different from the rest of the contact binaries. A comparison of an evolutionary model set with observations shows that both components are on the main sequence, the age of the binaries is at least several Gyr, while the contact phase lasts only less than 1 Gyr.

The results from a four-year systematic observational sky survey of eclipsing binaries, candidates for containing a delta Scuti component, are presented. More than a hundred systems were tested for a possible oscillating behavior, 17 out of which were found to show evidence of pulsating component and 8 more being ambiguous cases for further research. Data analysis and modelling methods are described step-by-step, and as an example the case of BO Her is presented in detail. Using the observational results of all known close binaries with a delta Scuti component collected from the literature and our own study, a total of 73 such systems is gathered. Correlations among their physical parameters (e.g. pulsation and orbital periods, evolutionary status) are also derived, extending significantly our knowledge of these systems.

We perform aperture photometry and profile fitting on 419 globular cluster (GC) candidates with mV <= 23 mag identified in Hubble Space Telescope/Advanced Camera for Surveys BVI imaging, and estimate the effective radii of the clusters. We identify 85 previously known spectroscopically confirmed clusters, and newly identify 136 objects as good cluster candidates within the 3σ color and size ranges defined by the spectroscopically confirmed clusters, yielding a total of 221 probable GCs. The luminosity function peak for the 221 probable GCs with estimated total dereddening applied is V ~ (20.26 ± 0.13) mag, corresponding to a distance of ~3.7 ± 0.3 Mpc. The blue and red GC candidates, and the metal-rich and metal-poor spectroscopically confirmed clusters, respectively, are similar in half-light radius. Red confirmed clusters are about 6% larger in median half-light radius than blue confirmed clusters, and red and blue good GC candidates are nearly identical in half-light radius. The total population of confirmed and "good" candidates shows an increase in half-light radius as a function of galactocentric distance. Based on observations with the Hubble Space Telescope obtained at the Space Telescope Science Institute, operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with Program GO-10250 and Program GO-10584.

New period of the third body in the system of the eclipsing HgMn binary star AR Aur was derived based on new photoelectric and CCD observations.

We present our ground-based CCD observations of the close binary systems DD Mon and XY UMa in B, V, R and I bands. The light curves are analyzed using the Wilson-Devinney code (W-D) for the derivation of the geometric and photometric elements of the systems. We compare the methods of photometric and spectroscopic mass ratio determination in these binaries, as a function of all typical difficulties, which arise during the analysis of such systems (light curve asymmetries, third light etc). Finally, a new spot model is suggested for the eclipsing system XY UMa, which belongs to the RS CVn type of active binaries.

The photometric and spectroscopic observational campaign organized for the 2008/9 eclipse of EE Cep revealed features, which indicate that the eclipsing disk in the EE Cep system has a multi-ring structure. We suggest that the gaps in the disk can be related to the possible planet formation.

As a result of the variability survey in χ Persei and NGC 6910, the number of β Cep stars that are members of these two open clusters is increased to twenty stars, nine in NGC 6910 and eleven in χ Persei. We compare pulsational properties, in particular the frequency spectra, of β Cep stars in both clusters and explain the differences in terms of the global parameters of the clusters. We also indicate that the more complicated pattern of the variability among B-type stars in χ Persei is very likely caused by higher rotational velocities of stars in this cluster. We conclude that the sample of pulsating stars in the two open clusters constitutes a very good starting point for the ensemble asteroseismology of β Cep-type stars and maybe also for other B-type pulsators.

We present results of the modelling of multicolour light curves of 10 contact binary systems: V376And, V523Cas, CCCom, BXDra, FGHya, UZLeo, XYLeo, AMLeo, EXLeo and RTLMi. The solutions resulted in a contact configuration for all systems. We found only FGHya and UZLeo to be in deep contact, the latter almost filling the outer critical lobe. The absolute parameters of the components have been determined with an accuracy of about a few per cent based on combined photometric and radial velocity curves, enlarging the sample of systems to 58 for which the physical parameters have been obtained in a uniform way. All but three systems (BXDra, AMLeo and RTLMi) show asymmetries and peculiarities in the observed light curves, interpreted as resulting from their magnetic activity.

The high quality stellar spectra, recorded with the stellar spectrograph constructed at the Laboratory of Astronomy and Applied Optics of the University of Athens, allows the spectral classification of stars brighter than 9 mag, as well as the measurement of radial velocities of binary systems. The primary application of the spectrograph will be the study of eclipsing binaries, where the lack of spectroscopic observations limits the theoretical models. The spectrograph can be also used for observations of transient phenomena, such as novae, supernovae, comets and cataclysmic variable stars, but also for support observations of larger telescopes around the world.

Three short period (P ∼ 1 day) variable stars from the Hipparcos catalogue targets were observed after suspected misclassification as Beta Lyr eclipsing systems (Perryman et al. 1997), as no secondary component had been noticed in the inspection of their Broadening Functions (BFs) (Rucinski 2002). FH Cam is found to be a multiple star system with a member exhibiting Delta Scuti behaviour. The dominant pulsation frequency is found to be 7.3411 ± 0.0002 c/d, which corresponds to a pulsation mode of l ≤ 1. We confirmed the pulsations of CU CVn using photometric observations and found a pulsation frequency of 14.7626 ± 0.0250 c/d, which is in agreement with the period given in literature. CC Lyn is a non-eclipsing visual binary (CCDM J07359+4302AB), the brighter component (A) is found to be a multi-mode Delta Scuti pulsator, with pulsation frequencies of 5.6402 ± 0.0004 c/d and 7.3368 ± 0.0005 c/d.

OJ 287 is a BL Lac object at redshift z = 0.306 that has shown double-peaked bursts at regular intervals of ~12yr during the last ~40yr. We analyse optical photopolarimetric monitoring data from 2005 to 2009, during which the latest double-peaked outburst occurred. The aim of this study is twofold: firstly, we aim to analyse variability patterns and statistical properties of the optical polarization light curve. We find a strong preferred position angle in optical polarization. The preferred position angle can be explained by separating the jet emission into two components: an optical polarization core and chaotic jet emission. The optical polarization core is stable on time-scales of years and can be explained as emission from an underlying quiescent jet component. The chaotic jet emission sometimes exhibits a circular movement in the Stokes plane. We find six such events, all on the time-scales of 10-20d. We interpret these events as a shock front moving forwards and backwards in the jet, swiping through a helical magnetic field. Secondly, we use our data to assess different binary black hole models proposed to explain the regularly appearing double-peaked bursts in OJ 287. We compose a list of requirements a model has to fulfil to explain the mysterious behaviour observed in OJ 287. The list includes not only characteristics of the light curve but also other properties of OJ 287, such as the black hole mass and restrictions on accretion flow properties. We rate all existing models using this list and conclude that none of the models is able to explain all observations. We discuss possible new explanations and propose a new approach to understanding OJ 287. We suggest that both the double-peaked bursts and the evolution of the optical polarization position angle could be explained as a sign of resonant accretion of magnetic field lines, a `magnetic breathing' of the disc.

These files present the observational data of the paper. There are 7 files : 6 for photometry (BVRIri) and one for polarimetry. Details of the reduction procedures and the telescopes involved can be found in the paper. (7 data files).

A new catalog of visual double systems containing eclipsing binaries as one component is presented. The main purpose of this catalog is to compile a complete list of all known multiples of this variety, both for current analysis and to highlight those in need of additional observations. All available photometric and astrometric data were analyzed, resulting in new orbits for eight systems and new times of minimum light for a number of the eclipsing binaries. Some of the systems in the catalog have acceptable solutions for their visual orbits, although in most cases their orbital periods are too long for simultaneous analysis. Also included, however, are a number of systems which currently lack an orbital solution but which may be suitable for simultaneous analysis in the future.

The blazar OJ 287 has produced two major optical outburst events during the years 2005-2008. These are the latest in a series of outbursts that have occurred repeatedly at 12 year intervals since early 1900s. It has been possible to explain the historical light curve fairly well by using a binary black hole model where the secondary black hole impacts the accretion disk of the primary twice during the 12 year orbital cycle. We will ask here how well does the latest light-curve fit with this model. We use a 10 million particle disk to model the accretion disk of the primary black hole. The rate of transfer of particles through the 10 Schwarzschild radius cylinder around the primary is followed. The secondary induces an inward flow through this surface. The inward flow rate is compared with the historical light curve as well as with the most recent observations reported in this paper. The observations have been carried out by using a number of small and medium size telescopes in different locations in order to ensure a dense light-curve coverage. The "inflow light curve" and the optical light curve of OJ 287 have a close resemblance to each other. It suggests that the tidally induced accretion flow is responsible for the main features of the optical light curve, with the exception of the quasi-periodic double peaks. It implies a close connection between the accretion disk and the jet where the optical synchrotron emission is presumably generated.

Physical parameters of contact binaries are studied through 2-D and 3-D correlations among them. It is shown that the physical parameters (i.e. mass, radius and luminosity) are closely correlated with the orbital period and mass ratio in the 3-D domain. These correlations can be used as a quality check for the parameters in every given solution of a contact binary. The empirical laws, extracted out of these correlations, are a useful tool for a quick estimate of physical parameters for the numerous contact binaries found in global sky surveys.

Six contact binaries lacking a period analysis have been chosen to search for the presence of a third body. The O-C diagrams of these binaries were analyzed with the least-squares method by using all available times of minima. Ten new minima times, obtained from our observations, were included in the present research. The Light-Time Effect was adopted for the first time as the main cause for the detailed description of the long-term period changes. Third bodies were found with orbital periods from 49 up to 100 years, and eccentricities from 0.0 to 0.56 for the selected binaries. In one case (WZ And), a fourth-body LITE variation was also applied. The mass functions and the minimal masses of such bodies were also calculated and a possible angular separation and magnitude differences were discussed for a prospective interferometric discovery of these bodies.

We systematically surveyed period variations of superhumps in SU UMa-type dwarf novae based on newly obtained data and past publications. In many systems, the evolution of the superhump period is found to be composed of three distinct stages: an early evolutionary stage with a longer superhump period, a middle stage with systematically varying periods, and a final stage with a shorter, stable superhump period. During the middle stage, many systems with superhump periods of less than 0.08 d show positive period derivatives. We present observational characteristics of these stages and give greatly improved statistics. Contrary to an earlier claim, we found no clear evidence for a variation of period derivatives among different superoutbursts of the same object. We present an interpretation that the lengthening of the superhump period is a result of the outward propagation of an eccentricity wave, which is limited by the radius near the tidal truncation. We interpret that late-stage superhumps are rejuvenated excitation of a 3:1 resonance when superhumps in the outer disk are effectively quenched. The general behavior of the period variation, particularly in systems with short orbital periods, appears to follow a scenario proposed in Kato, Maehara, and Monard (2008, PASJ, 60, L23). We also present an observational summary of WZ Sge-type dwarf novae. Many of them have shown long-enduring superhumps during a post-superoutburst stage having longer periods than those during the main superoutburst. The period derivatives in WZ Sge-type dwarf novae are found to be strongly correlated with the fractional superhump excess, or consequently with the mass ratio. WZ Sge-type dwarf novae with a long-lasting rebrightening or with multiple rebrightenings tend to have smaller period derivatives, and are excellent candidates for those systems around or after the period minimum of evolution of cataclysmic variables.

We present results on the X-ray source populations of nearby spiral and star-forming galaxies. The sample of galaxies includes the nearby spiral galaxy M81 and star-forming galaxies with stellar populations in the 10-100Myr range. We use HST data in order to classify the X-ray sources to HMXBs, LMXBs and globular clusters, and identify any SNRs or background AGNs. We discuss the spectral and temporal characteristics of the different sub-populations in these galaxies, and their implications for the sources' nature. We present the X-ray luminosity functions (XLF) of the discrete sources from the individual as well as the co-added observations and we discuss the dependence of the XLF on the age of the stellar populations. In the case of M81 we find a population of sources in the 10^{36}-10^{37} erg s^{-1} luminosity range with early-type counterparts, while the HMXBs in the star-forming galaxies have generally higher luminosities. We also present results from Chandra observations of the SMC which extend the XLFs down to luminosities of 10^{32} erg s^{-1} and show evidence for the onset of the propeller effect at low luminosities.

CCD photometric observations of the Algol-type eclipsing binary TZ Eri have been obtained in B and V filters during 26 nights from December 2007 to February 2008 at the Athens University Observatory. The light curves are analyzed with the Wilson-Devinney code, new geometric and photometric elements are derived, a time series analysis of the observations is applied and a multiperiodic behavior is also discussed. The presence of a third light in the system is considered and our results are compared with those of the O-C analysis for a third body in the system, given by Zasche et al. (2008).

Stars with changing Blazhko periods challenge the currently proposed hypotheses for the Blazhko effect. RR Lyr, the prototype of the class, is one of the best-studied Blazhko stars but it keeps on surprising its observers. We present the first results from a photometric follow- up campaign in 2006-2007 of the star. Multicolour data were gathered from 4 different observatories in the northern hemisphere. Our analysis focuses specifically on the period behaviour. We confirm the previously reported decrease of the modulation period.

Various scenarios of contact binary evolution have been proposed in the past, giving hints of (sometimes contradictory) evolutionary sequences connecting A- and W-type systems. As the components of close detached binaries approach each other and contact binaries are formed, following evolutionary paths transforms them into systems of two categories: A-type and W-type. The systems evolve in a similar way but under slightly different circumstances. The mass/energy transfer rate is different, leading to quite different evolutionary results. An alternative scenario of evolution in contact is presented and discussed, based on the observational data of over one hundred low-temperature contact binaries. It results from the observed correlations among contact binary physical and orbital parameters. Theoretical tracks are computed assuming angular momentum loss from a system via stellar wind, accompanied by mass transfer from an advanced evolutionary secondary to the main-sequence primary. A good agreement is seen between the tracks and the observed graphs. Independently of details of the evolution in contact and a relation between A- and W-type systems, the ultimate fate of contact binaries involves the coalescence of both components into a single fast rotating star.

An alternative to TRO model of a W UMa-type star is presented in which the binary is past mass exchange with mass ratio reversal. The secondary is hydrogen depleted and both components are in thermal equilibrium. Evolution in contact is driven by orbital angular momentum loss and mass transfer from the secondary to primary component, similarly as it is observed in Algols. Temperature equalization of both components results from an assumed energy transfer by a large scale flow encircling the whole system in the common envelope.

We present preliminary results from a study of the X-ray source population and the star formation history (SFH) of the nearby galaxy M81, based on archival Chandra data and our HST/ACS survey. These data provide the deepest and most detailed picture of the stellar populations in this prototypical spiral galaxy. The preliminary analysis of the ACS data shows variations of the SFH across the galaxy. The Chandra observations allow us to classify the X-ray sources and study their link with the SFH. In combination with the ACS data we can identify their optical counterparts and classify them, based on their photometric properties. We can classify 80% of the detected X-ray sources as OB stars, globular clusters, giant/supergiant stars, and background AGNs, demonstrating the diagnostic power of combining high spatial resolution optical and X-ray observations.

We present the first results from a study of the X-ray source populations in M81 in the context of their local star-formation histories. We use deep Chandra observations and our multi-band HST maps of M81 in order to detect and classify the X-ray sources, based on their X-ray and optical properties. We compare the different X-ray source populations (Luminosity functions and relative numbers of different source types) for the different regions of the galaxy and for stellar populations in different age ranges (derived from the HST data). This way we can identify High-mass X-ray binaries with Main Sequence and supergiant donors, and we find that their luminosity functions are distinctly different. We also construct the luminosity functions of the X-ray sources associated with globular clusters. Finally we present our results from the spatially resolved star-formation history of M81, based on the study of the color-magnitude diagrams in different regions of the galaxy. We discuss these results in the context of similar studies of star-forming galaxies spanning a wide range of starburst ages.

Context: In the H-R diagram, the intersection of the main sequence and the classical Cepheid instability strip corresponds to a domain where a rich variety of atmospheric phenomena are at play (including pulsation, radiative diffusion, convection). Main-sequence A-type stars are among the best candidates to study the complex interplay between these various phenomena. Aims: We have explored a sample of suspected A-type binaries in a systematic way, both spectroscopically and photometrically. The sample consists of main-sequence A-type stars for which the few existing radial velocity measurements may show variability, but for which other essential information is lacking. Due to their location in the H-R diagram, indications of pulsation and/or chemical peculiarities among these suspected binary (or multiple) systems may be found. Methods: High-resolution spectroscopy obtained with the ELODIE and MUSICOS spectrographs was used in combination with a few nights of differential CCD photometry in order to search for pulsation(s). In order to search as well for chemical peculiarities or for possible hidden component(s), we derived the atmospheric stellar parameters by fitting the observed spectra with LTE synthetic ones. Results: Of the 32 investigated targets, eight are spectroscopic binaries, one of which is a close binary also showing eclipses, and three have been identified as δ Scuti pulsators with rapid line-profile variations. Conclusions: Among the latter stars, HD 217860 reveals interesting multiperiodic photometric and spectroscopic variations, with up to eight frequencies common to two large photometric data sets. We suggest that at least one radial overtone mode is excited among the two most dominant frequencies, on the basis of the computation of the pulsation constants as well as of the predicted frequencies and the expected behaviour of the amplitude ratio and the phase difference in two passbands using adequate theoretical modelling. We furthermore found evidence for a strong modulation of the amplitude(s) and/or the (radial) frequency content of this intriguing δ Scuti star. This work is based on spectroscopic observations made at the Haute-Provence Observatory (OHP), the Observatoire du Pic du Midi (TBL) and the Bulgarian National Astronomical Observatory (NAO, Rozhen). Tables 1, 2, 4, 5 and Fig. 7 are only available in electronic form at http://www.aanda.org

New BVRI CCD observations of the contact binary system HV~Aqr are presented. The new light curves are analyzed with the WD code and new geometric and photometric elements are derived. These elements are combined with the available spectroscopic data to yield the physical parameters of the system. The agreement between the spectroscopically and photometrically determined mass ratios of eclipsing binaries is also discussed.

We present the initial results from a deep HST survey of the nearby spiral galaxy M81 with the HST Advanced Camera for Surveys. The survey consists of B and V band observations of 29 ACS fields, totaling to 38 orbits. These observations, together with archival I-band ACS data, provide complete, deep coverage of the D25 area of the galaxy, down to 26-27 mag. This detection limit probes the bulk of early-type (O,B) stars and the population of star-clusters. The main goal of this survey is to characterize the X-ray source population of M81 in the context of its stellar populations and star-formation history. We discuss the analysis of the ACS data (mosaicing, astrometric registration, photometric analysis) and we present high quality images of the galaxy. We also discuss the initial results from the identification of optical counterparts to the X-ray sources detected in deep Chandra observations of M81, and the preliminary study of the stellar populations in individual fields of the galaxy. This work is supported by HST grant HST-GO-10584.01-A and NASA LTSA grant NAG-5-13056.

Radial velocity measurements and sine-curve fits to the orbital radial velocity variations are presented for 10 close binary systems: OO Aql, CC Com, V345 Gem, XY Leo, AM Leo, V1010 Oph, V2612 Oph, XX Sex, W UMa, and XY UMa. Most of these binaries have been observed spectroscopically before, but our data are of higher quality and consistency than in the previous studies. While most of the studied eclipsing pairs are contact binaries, V1010 Oph is probably a detached or semidetached double-lined binary, and XY UMa is a detached, chromospherically active system whose broadening functions clearly show well-defined and localized dark spots on the primary component. A particularly interesting case is XY Leo, which is a member of visually unresolved quadruple system composed of a contact binary and a detached, noneclipsing, active binary with an 0.805 day orbital period. V345 Gem and AM Leo are known members of visual binaries. We found faint visual companions at about 2"-3" from XX Sex and XY UMa. Based on data obtained at the David Dunlap Observatory, University of Toronto.

We report on the results of a series of photometric observations of the low-amplitude δ Scuti star GSC 4778:0324, in the course of a multi-site campaign aiming at monitoring V1162 Ori. The two dominant frequencies, found in the previous analyses based on the 2000-2002 data, are confirmed and their amplitudes are found to be stable on a time-scale of a few years.

The first complete CCD light curves of the eclipsing binary systems AX Virginis and V1034 Cygni have been obtained in the B, V, R, I and V, R, I filters, respectively, during the spring and summer of 2004 with the 40-cm telescope of the University of Athens Observatory. The light curves were analyzed with the W-D program in order to determine the geometrical and physical parameters of the systems.

Results are presented on component masses and system angular momenta for over 100 low-temperature contact binaries. It is found that the secondary components in close binary systems are very similar in mass. Our observational evidence strongly supports the argument that the evolutionary process goes from near-contact binaries to A-type contact binaries, without any need of mass loss from the system. Furthermore, the evolutionary direction of A-type into W-type systems with a simultaneous mass and angular momentum loss is also discussed. The opposite direction of evolution seems to be unlikely, since it requires an increase of the total mass and the angular momentum of the system.

New high-quality CCD photometric light curves for the W UMa-type systems V410 Aur, CK Boo, FP Boo, V921 Her, ET Leo, XZ Leo, V839 Oph, V2357 Oph, AQ Psc and VY Sex are presented. The new multicolor light curves, combined with the spectroscopic data recently obtained at David Dunlap Observatory, are analyzed with the Wilson-Devinney code to yield the physical parameters (masses, radii and luminosities) of the components. Our models for all ten systems resulted in a contact configuration. Four binaries (V921 Her, XZ Leo, V2357 Oph and VY Sex) have low, while two (V410 Aur and CK Boo) have high fill-out factors. FP Boo, ET Leo, V839 Oph and AQ Psc have medium values of the fill-out factor. Three of the systems (FP Boo, V921 Her and XZ Leo) have very bright primaries as a result of their high temperatures and large radii.

We present the results of extensive time-series photometry of the pulsating subdwarf B star KPD 2109 + 4401. Our data set consists of 29 data runs with a total length of 182.6h over 31d, collected at five observatories in 2004. These data comprised high signal-to-noise ratio observations acquired with larger telescopes and wider time-coverage observations obtained with smaller telescopes. They are sufficient to resolve the pulsation structure to 0.4 μHz and are the most extensive data set for this star to date. With these data, we identify eight pulsation frequencies extending from 4701 to 5481μHz, corresponding to periods of 182-213s. The pulsation frequencies and their amplitudes are examined over several time-scales with some frequencies showing amplitude variability.

The extensive long-term CCD and photoelectric photometric observations of the classical symbiotic star YY Her covering the period of its post-outburst activity (JD 2 451 823 -- 2 452 996) are presented. We explain the periodic variations of the brightness of YY Her by the eclipses of the components in the symbiotic system. The model with a deformed (non-homogeneous) envelope, surrounding the white dwarf is discussed. In addition, we observed a flare in about JD 2 452 440, during the primary minimum, that was later followed by an energetic outburst in JD 2 452 700.

We present follow-up observations of pulsating sdB stars as part of our efforts to resolve the pulsation spectra for use in asteroseismological analyses. This paper reports on our overall efforts, but specifically on our results for the pulsating sdB stars KPD 2109+4401 and PG 0154+182.

We present follow-up observations of pulsating sdB stars as part of our efforts to resolve the pulsation spectra for use in asteroseismological analyses. This paper reports on our overall efforts, but specifically on our results for the pulsating sdB stars PG 1618+563 and EC 05217-3914.

The first VRI CCD photometric observations of the newly discovered ROTSE-1 eclipsing variable GSC 2534:1121 are presented. Our new times of minima are used along with those of the last two years to calculate a new ephemeris of the system. The observed light curves are analyzed with the Wilson-Devinney program in order to derive the geometric and photometric elements of the system. The moderate O'Connell effect is explained by assuming a cool spot on the primary component.

Seven new precise times of minimum light have been gathered for the triple eccentric eclipsing binary YY Sgr (P = 2d.63, e = 0.16). Its O--C diagram is presented and improved elements of the apsidal motion and the light-time effect are given. We found a new short period of the third body of about 18.5 years in an eccentric orbit (e 3 ≃ 0.4).

During recent observations it was possible to increase the field of view for the Cassegrain telescope at the Kryoneri Astronomical Station of the National Observatory of Athens. The very small area of sky observable with the existing CCD, is a problem when larger fields need to be observed. A focal reducer was installed on the telescope thus increasing the observed area by a factor of about 4. The advantage is faster photometry and detection of very faint stars (>20 mag). A permanent focal reducer should be mounted on the telescope for the benefit of all observers.

New precise times of minimum light for several early-type eclipsing binaries were obtained at three observatories. The changes of period of the following measured binaries are discussed: V1182 Aql, LY Aur, SZ Cam, FZ CMa, QZ Car, LZ Cen, V606 Cen, AH Cep and TU~Mus.

Orbital period changes of the eclipsing binaries GO Cyg and GW Cep are explained by the light-time effect for the first time. New minima of the eclipsing binary AR Aur improve the predicted light-time orbit. The light-time orbit with the quadratic ephemeris of the binary matches the new observations of V505 Sgr better than the linear one. As the light-time effect fits in corresponding O - C diagrams of all four systems have been reaching extreme values, the observations of minima times in forthcoming years are highly desirable.

Time-series CCD photometry of the contact system V839 Oph on May, June and July 2004 showed a continuous brightening of the system in all phases of the light curve. A possible explanation of this brightening could be enhanced (magnetic) activity of the system, also noticed in the past. Mass transfer and/or energy exchange between the two components can explain the observed increase of brightness. All possible cases, concerning the location and shape of the spotted area, are discussed and the most probable model is presented.

A new sample of contact systems, consisting of more than 100 stars, was created for binaries for which the physical parameters have been determined using both photometric light curves and radial velocity measurements of both components. Properties of components are discussed including their evolutionary status.

The first BVRI CCD photometric observations of PY Lyrae were obtained at the Kryoneri Observatory in the summer of 2004. Five new times of minima and a new ephemeris were calculated. The light curves were analyzed with the Wilson-Devinney program, in order to derive the geometric and photometric elements of the system. A spot model was assumed in order to explain the pronounced O'Connell effect.

We present a test-study of the ``seeing'' at the Kryonerion Station of the National Observatory of Athens. We used a Differential Image Motion Monitor (DIMM) during July and October 2002 and found a median ``seeing'' of 0.68 arcseconds and 1.42 arcseconds, respectively. We also compared the technique against the Hartmann-version of the ESO-type DIMM method and found similar results within 0.05 arcseconds. For some reason, which is under investigation by international teams, the measurements of our ESO-type DIMM are underestimated by ~ 10% compared to ESO-standard DIMM measurements. Simultaneous ESO-type DIMM measurements and standard gauss-fitting-technique measurements, taken with the Kryonerion 1.2m telescope (inside the dome) indicate that the ``seeing'', as measured by the telescope is by 1 arcsecond larger. The above tests were performed in order to calibrate, test and make a brief sampling of the ``seeing'' at the Kryonerion Station with the aim to move eventually the equipment to the Emilios Harlaftis Astronomical Station the site of the new 2.3m ARISTARCHOS telescope - for a long-term ``seeing'' monitoring. As a by-product of this project, a new DIMM Station became operational at Mt. Holomon, in the premises of the University of Thessaloniki, in 2004.

We present the results of a statistical investigation of the period-color and period-bolometric magnitude relations using a carefully selected sample of 120 contact systems with known physical parameters.

Aims.Extensive photometry of RR Lyr was obtained over a 421-day interval in 2003-2004, covering more than 10 Blazhko cycles in a multisite campaign. The length and density of this data set allow for a detailed analysis. Methods: .We used Fourier techniques to study RR Lyr's behavior over the pulsation and the Blazhko cycle. We propose a two-frequency model for decomposing the frequency spectrum. Results: .The light variations were fitted with the main radial frequency, its harmonics up to 11th order, and the detected triplet frequencies. No significant quintuplet components were found in the frequency spectrum. Given the total time span of the measurements, we can now unambiguously conclude that the Blazhko period has become notably shorter than the previously known value of 40.8 days, whereas the main pulsation period remained roughly the same. Changes in the modulation period have been reported for other well-studied Blazhko variables. They challenge the explanations for the Blazhko effect which link the modulation period directly to the rotation period. The new photometry reveals an interval in the pulsation cycle of RR Lyr during which the star's intensity barely changes over the Blazhko cycle. This interval occurs during the infalling motion and between the supposed phases of the early and the main shock. The data also permit a more detailed study of the light curve shape at different phases in the Blazhko period through Fourier parameters.

The long-term CCD and photoelectric observations of YY Her covering the period of its post-outburst activity (JD 2 451 823 -- 2 452 996) are presented. We explain the periodic variations of the brightness of YY Her by the eclipses of the components in the system. The model with a deformed (non-homogeneous) envelope, surrounding the white dwarf is discussed. In addition, we observed a flare in about JD 2 452 440, during the primary minimum, that was later followed by an energetic outburst in JD 2 452 700.

We present and discuss the Scargle periodograms of fast-speed B-band optical photometry of the eclipsing dwarf nova EX Draconis during quiescence and outburst, obtained with the 1.2m telescope of the National Observatory of Athens (NOA) in July 2000. We report on the discovery of a 143 s periodicity which most likely is related to the spin period of the white dwarf. We also find periodicities near 115 and 123 seconds during outburst which may be caused by pulsations of the accreting white dwarf in response to the enhanced mass transfer rate.

The outburst and eclipse characteristics of the eclipsing dwarf nova EX Dra prompted us to monitor it during the months July-August 2000 with the 1.2m telescope of the National Observatory of Athens (NOA). Here, we present an eclipse mapping study of the July 2000 light curves. B-band of the August 2000 data have been presented by Harlaftis et al. (2002).

New CCD observations of the pulsating star SZ Lyn are analyzed and a new ephemeris is proposed. The photometric data collected over the last 43 years are used to determine and update the pulsational and orbital elements, based on the light-time effect detected in the O-C diagram.

New ground-based BV RI CCD observations of the eclipsing binary systems NN Vir and YY CrB are analyzed and illustrated. New times of minima are given and new ephemerides are proposed. The light curves are analyzed with the Wilson-Devinney light curve synthesis code and new geometric and photometric elements are derived. These elements are used together with the available spectroscopic data to compute absolute elements. The evolutionary status of each system is studied by means of mass-radius diagrams. The systems NN Vir and YY CrB are found to be A-type and W-type W UMa systems, respectively.

We report multicolour photometric observations of the 2003 eclipse of the long-period (5.6 yr) eclipsing binary EE Cep. Measurements were obtained with ten telescopes at eight observatories in four countries. In most cases, UBV(RI)C broad band filters have been used. The light curve shape shows that the obscuring body is an almost dark disk around a low-luminosity central object. However, variations of the colour indices during the eclipse indicate that the obscuring body emits a considerable amount of radiation in the near infrared.

The paper presents new geometric, photometric and absolute parameters, derived from combined spectroscopic and photometric solutions, for ten contact binary systems. The analysis shows that three systems (EF Boo, GM Dra and SW Lac) are of W-type with shallow to moderate contact. Seven systems (V417 Aql, AH Aur, YY CrB, UX Eri, DZ Psc, GR Vir and NN Vir) are of A-type in a deep contact configuration. For six systems (V417 Aql, YY CrB, GM Dra, UX Eri, SW Lac and GR Vir) a spot model is introduced to explain the O'Connell effect in their light curves. The photometric and geometric elements of the systems are combined with the spectroscopic data taken at David Dunlap Observatory to yield the absolute parameters of the components.

New BVRI light curves of the eclipsing binary system V1363 Ori, based on CCD observations made at the University of Athens Observatory, are analyzed using the Wilson-Devinney light curve synthesis code and new geometric and photometric elements are derived. These elements are used together with the available spectroscopic data to compute the absolute elements of the system. The evolutionary status of the system is also discussed.

The physical parameters of ten eclipsing binaries (eight contact and two semi-detached) are derived by using ground-based photometric observations and Hipparcos/Tycho photometric data, which mimic the photometric observations that are expected to be obtained by Gaia. The results are compared and the achievable precision of the basic stellar parameters derived by Gaia photometry is discussed.

The nova-like variable TT Ari had shown a luminosity change with a drastic change of the light curve. This state is distincly different from both the bright "negative superhump" state and the "positive superhump" state known before.

The double-mode pulsation of GSC 00144-03031 has been detected when searching for COROT targets. A very large dataset composed of 4722 photometric measurements was collected at six observatories in Europe and America. There is no hint of the excitation of additional modes (down to 0.6 mmag) and therefore GSC 00144-03031 seems to be a pure double-mode pulsator, with a very short fundamental radial mode (P = 84 min). From uvbyβ photometry and evolutionary tracks it appears to be a Pop. I star with M = 1.75 M⊙, located in the middle of the instability strip, close to the Zero-Age Main Sequence. We also discovered other new double-mode pulsators in the databases of large-scale projects: OGLE BW2_V142, OGLE BW1_V207, ASAS3 094303-1707.3, ASAS3 000116-6037.0, NSVS 3234596 and NSVS 3324715. An observational Petersen diagram is presented and explained by means of new models. A common sequence connecting Pop. I stars from the shortest to the longest periods is proposed and the spreads in the period ratios are ascribed to different metallicities (at the shortest periods) and to different masses (at the longest ones).

During two observational seasons of international campaign of YY Her we secured photometric data which covered the primary as well as the secondary minima and revealed unexpected outburst activity of this system.

New BVRI CCD observations of the pulsating star SZ Lyn are presented. The light curves are analyzed using Fourier transform techniques and a new period is determined. A new ephemeris, based on recent times of maxima, is proposed. The photometric data collected over the last 40 years are used to determine the pulsational and orbital elements, since SZ Lyncis is a member of a binary system. The results are compared with those found in previous studies.

SZ Lyncis (=HD 67390) is a high amplitude Delta Scuti star, discovered by Hoffmeister in 1949. Van Genderen (1967) noticed that the residuals in the O-C diagrams followed a sinusoidal variation with a period of Porb = 3.091 years. Several years later, Barnes & Moffett (1975) improved this period to Porb = 3.138 years and suggested that the sinusoidal variations in the O-C diagrams, observed by van Genderen (1967), originate from the light travel time effect, since SZ Lyn is a member of a binary system. This hypothesis was confirmed by Bardin & Imbert (1984) with measurements of radial velocities. Soliman et al. (1986) found the pulsation period to be 0.120534896 days and the orbital period 1173.5 days.

Radial velocity measurements and sine-curve fits to the orbital velocity variations are presented for the eighth set of 10 close binary systems: AB And, V402 Aur, V445 Cep, V2082 Cyg, BX Dra, V918 Her, V502 Oph, V1363 Ori, KP Peg, and V335 Peg. Half of the systems (V445 Cep, V2082 Cyg, V918 Her, V1363 Ori, and V335 Peg) were discovered photometrically by the Hipparcos mission, and all systems are double-lined (SB2) contact binaries. The broadening function method permitted improvement of the orbital elements for AB And and V502 Oph. The other systems have been observed for radial velocity variations for the first time; in this group are five bright (V<7.5) binaries: V445 Cep, V2082 Cyg, V918 Her, KP Peg, and V335 Peg. Several of the studied systems are prime candidates for combined light and radial velocity synthesis solutions. Based on the data obtained at the David Dunlap Observatory, University of Toronto.

The first l VRI CCD photometric observations of the eclipsing binary system OT Cep are presented. The new times of minima, combined with the existing ones, are used to determine a new ephemeris of the system. Our light curves are analyzed with light curve synthesis techniques to derive the first geometric and photometric elements of OT Cep. The possible evolutionary status of the system is also discussed.

New l BVRI CCD observations of the eclipsing binary NSV 223 are described, and a new ephemeris is proposed. The light curves are analyzed using the Wilson-Devinney light curve synthesis code, and the new geometric and photometric elements are derived. These elements, combined with the available spectroscopic data, yield the absolute system parameters, which make it possible to construct a model of the system.

This paper reports new photometric observations of minima of two very long-period eclipsing systems: AZ Cas (P=3402 °}) and EE Cep (P=2050 °). In order to match the observed depth of the primary minimum of AZ Cas we had to introduce a third light component. For EE Cep, the shape of the eclipse can be explained by a model that includes a dark precessing disk in the system.

The paper presents combined spectroscopic and photometric orbital solutions for five close binary systems: V 402Aur, SX Crv, V829 Her, VZ Lib and V753 Mon. The photometric data consist of new complete, multicolor light curves, while the spectroscopy has been recently obtained within the radial velocity program at the David Dunlap Observatory. For one target, SX Crv, new spectroscopic data were obtained using the 6.5 m Magellan telescope. A contact configuration was found through light curve modeling for all targets except V753 Mon. Our solution for V753 Mon resulted in a semidetached configuration with the slightly less massive component filling its Roche lobe.

The paper presents combined spectroscopic and photometric orbital solutions for four close binary systems: SW Lyn, QW Gem, AP Leo and V2150 Cyg. The photometric data are new, while the spectroscopy has been recently obtained within the radial velocity programme at the David Dunlap Observatory. This paper is the first in the planned series of investigations. We give an extensive description of the motivation for the series and of the main assumptions made in our solutions. The four targets of this investigation span a range of typical configurations and thus present different levels of difficulty for the combined spectroscopic and photometric orbital solutions.

We report the results of the second large campaign of photometric observations for the small-amplitude delta Scuti star GSC 4778-00324, detected in the course of a multi-site campaign aiming at monitoring V1162 Ori. We confirm the presence of the same two frequencies as found in the first analysis based on the 2000-2001 data.

Recent highlights of our study of positive and negative superhumps in cataclysmic varibles are briefly reviewed. The ``genuine" superhumps have been detected a day after the unexpected superoutburst of WZ Sge, when the orbital variability dominated the light curve. TT Ari has shown an unprecedented switch form the usual negative superhump state to the positive state with an excellent coincidence between the observed and theoretically predicted period values. Both stars show evidence for variability at a time scale of 20 minutes. The superhump period of RZ LMi has been corrected based on the observations of the superoutburst in 2001. ES Dra shows no evidence for prominent superhumps, possibly indicationg its low inclination. However, the superoutburst has shown a splitted maximum with a separation of 20 days. The ``superhump period-luminosity" dependence has been detected for two cataclysmic variables of different types -- the nova-like BZ Cam and the UG SU star V368 Peg.

The first complete I-filter light curve of the eclipsing sdB binary HS 0705+6700 is analyzed using the Wilson-Devinney code and new geometric and photometric elements are derived. Using these elements together with the available spectroscopic data, it was possible to build a detailed model of the system. Our results are in good agreement with those found from photometric studies in other bands.

Optical Interferometry can play an important role in modern astronomy. Extrasolar planet and binary star research, are some of the fields, where we will face important discoveries in the future. For the VLTI this will be a real challenge, since it will be the pioneer in angular resolution. In addition, high flux sensitivity will help detecting not only Jupiter-sized planets, but Earth-sized planets too. The ability of the VLTI in detecting new planets, measuring stellar diameters and stellar proper motions, with the use of such techniques, is presented.

We report on time-series photometric observations in the earliest stages of superoutbursts of the extreme dwarf novae, AL Com and WZ Sge, which started on 2001 May after the 6 years quiescence and on 2001 July after the 23 years quiescence, respectively. We detected the growth of ``early superhumps'' during the each rising stage. Our observations reject the mass transfer instability for the trigger of the superoutburst of WZ Sge stars, and show the existence of some relations between the ``early superhumps'' and the spiral structure, which gives a hint of the origin of the ``early superhumps.''

We observed EX Draconis in BV I colours for 40 nights, covering 3 outburst events, with the aim to study the disc evolution. Here, we present a preliminary eclipse-mapping study of some of the B band light curves obtained during the outburst in August 2000.

After the unexpected superoutburst of WZ Sge on July 23, 2001, the light curve was dominated by the orbital variability with a switch to dominating superhumps after August 5. The time series analysis of our data has shown, that the superhumps have started at the early stage of the superoutburst.

We analyse the fine structure of the type IV radio burst, during the main phase of the 14th July 2000 major event recorded by the radiospectrograph ARTEMIS-IV, in the 110-650 MHz (10 Hz sampling rate) and the 270-450 MHz (100 Hz rate) range. In this report we focus on variations in frequency drift rate and repetition rate of fibers bursts. A pronounced asymmetry between the number of bursts with positive and negative frequency drift is noted at certain time periods. These variations follow closely the evolution of the magnetic structure associated with the major solar event and the onset of the corresponding CME.

The dwarf nova EX Draconis shows deep and wide eclipses, in addition to its frequent outburst activity (every 2-3 weeks), which makes it the ideal target for monitoring a complete outburst cycle. We observed EX Draconis in BVI colours with the 1.2m telescope at Kryonerion Korinthias (Greece) between 1 July-20 August 2000 covering 3 outburst events (1-5 July, 20-30 July and 6-12 August). Here, we present a preliminary eclipse mapping study of the light curves obtained in August 2000.

We present the results of multisite observations of the delta Scuti star V 1162 Ori. The observations were done in the period October 1999-May 2000, when 18 telescopes at 15 observatories were used to collect 253 light extrema during a total of 290 hours of time-series observations. The purpose of the observations was to investigate amplitude and period variability previously observed in this star, and to search for low-amplitude frequencies. We detect, apart from the main frequency and its two first harmonics, four additional frequencies in the light curves, all with low amplitudes (1-3 mmag). Combining the present data set with data obtained in 1998-99 at ESO confirms the new frequencies and reveals the probable presence of yet another pulsational frequency. All five low-amplitude frequencies are statistically significant in the data, but at least one of them (f5) suffers from uncertainty due to aliasing. Using colour photometry we find evidence for a radial main frequency (f1), while most or all low-amplitude frequencies are likely non-radial. We show that the main frequency of V 1162 Ori has variable amplitude and period/phase, the latter is also displayed in the O-C diagram from light extrema. The amplitude variability in our data is cyclic with a period of 282 d and a range of nearly 20 mmag, but earlier amplitude values quoted in the literature cannot be explained by this cyclic variation. O-C analysis including data from the literature show that the period of V 1162 Ori displays a linear period change as well as sudden or cyclic variations on a time scale similar to that of the amplitude variations. Based on observations obtained at the South African Astronomical Observatory (SAAO), Athens University and Kryonerion Observatories, European Southern Observatories (ESO: applications ESO 62H-0110, 64H-0065 and 64L-0182), Laboratório Nacional de Astrofísica (Brazil), Xinglong, Beersel Hills, Ege University, San Pedro Martir, Merate, Mt. Laguna, Siding Spring, Sierra Nevada, Braeside and Lick Observatories. Table 2 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/374/1056

V1162 Ori is a delta Scuti star with variable period and amplitude. To analyse data acquired in the years 1998-2000, the data were subdivided in smaller subsets to take the amplitude and period variability into account. Four different ways of subdividing the data discussed in the paper are outlined in this table 2. A database containing the observations of V1162 Ori is in preparation; it will be published in the Journal of Astronomical data (http://www.vub.ac.be/STER/JAD/jad.htm). (2 data files).

In 1999 the University of Athens installed a 0.4-m Cassegrain telescope (CCT-16, by DFM Engineering) on the roof of the Department of Astrophysics, Astronomy and Mechanics, equipped with a ST-8 CCD camera and Bessel UBVRI filters. Although the telescope was built for educational purposes, we found it can be a perfect research instrument, as we can obtain fine quality light curves of bright variable stars, even from a place close to the city center. Light curves of the δ Scuti star V1162 Ori and of the sdB star PG 1336-018 are presented, showing the ability of a 40-cm telescope to detect negligible luminosity fluctuations of relatively bright variable stars. To date, we succeed in making photometry of stars down to 15th magnitude with satisfactory results. We expect to achieve even better results in the future, as our methods still improve, and as the large number of relatively bright stars gives us the chance to study various fields of CCD photometry of variables.

Time series analysis of 6 nights of CCD observations of WZ Sge after it's unexpected superoutburst on July 23, 2001 has shown highly asymmetric periodic variations with a period 0.0566513(22)deg, full amplitude 0.218(4)m and a very high asymmetry M-m=0.670(9), a secondary minimum 0.38P after the main one. The initial epochs are Tmax=BJD 2452118.7483(3) and Tmin=BJD 52118.7670(2). The characteristics of the phase light curve are listed. In addition, we have found superhumps with a semi-amplitude of 0.021(2)m and an ephemeris Tmax,sh=BJD 2452117.6830(8)+0.057435(45)d E. Other peaks at 21 and 25 minutes possibly correspond to quasi-periodic oscillations with an effective semi-amplitude of 0.014m.

We report that the intermediate amplitude delta Scuti star V 1162 Ori has changed its main pulsational period in the course of the year 2000. This new period change falls in a sequence of period changes observed during the last 5 years. While the average amplitude value of all our new data, 63 mmag, fits a cyclic amplitude variation suggested by Arentoft et al. (2001), splitting the data up in smaller subsets discloses significant deviations from regularity, with stretches of constant amplitude during short intervals of time. The new data show that the amplitude of one of the secondary frequencies, f2, has in 3 years dropped from more than 3 mmag to now about 1 mmag, and that the previously obtained f5 probably is a 1 d-1 alias of the real frequency. We present the newly acquired times of minimum and maximum light as support for subsequent observing campaigns. Based on observations obtained at the South African Astronomical Observatory (SAAO), Athens University Observatory, the Danish 1.5 m telescope at ESO, La Silla, Chile, and Beersel Hills Observatory.

The cataclysmic variable, WZ Sge, and one of prototype of of the SU Uma-type dwarf nova sub-class, went into outburst on July 23 of this year. This is its first outburst since 23 years. The brightness and rarety of the event has perhaps made it the most observed object of the year. From July 29 on, we have started a photometric monitoring of this system, mainly with B-band time series with the 1.2m telescope at Kryoneri Korinthias (Greece). Here, we will present the first preliminary light curves obtained, covering the first two and a half weeks of the outburst, from July 29 to August 9, when the star was in steady decline up to when it started to show superhumps. Our on-going monitoring will continue until December 2001.

Recent results obtained within the framework of the international observational and research campaigns are reviewed. The main topics are following: magnetic cataclysmic variables: synchronization of the spin and orbital period of the white dwarf; observational evidence on the switching of the accretion from one pole to another; non-dipole configuration of the magnetic field; ``swingings'' of the orientation of the magnetic axis in the `` synchronous systems''; instability of the accretion and the ``red noise''; long-term luminosity changes; UV Cet - like flares; two-pole accretion in the intermediate polars; weakly- and non- magnetic cataclysmic variables: negative and positive superhumps in the nova-like variables; ``red noise'' and/or quasi-periodic oscillations; unprecedented switch of the superhump period in TT Ari; the luminosity dependence of the superhump period in BZ Cam and V725 Aql; long-term variations of the outburst characteristics of dwarf novae and luminosity of old novae - magnetic activity of the red secondary vs third body orbiting around a cataclysmic variable; symbiotic binaries: light curve variations in symbiotic novae and stars with a pulsating component; long-periodic pulsating variables: statistical study of the characteristics of the mean phase curves and of the individual cycles; additional classification criteria for the Mira-type and semi-regular stars. Some of the original papers are available at http://www.paco.odessa.ua/~il-a and http://ila.webjump.com