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.

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.

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.

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.

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.

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 PG 1618+563 and EC 05217-3914.

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.

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.

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.

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.

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.

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).

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.

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.

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.