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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, RWCom, 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 ultra-short 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 confgurations even after several Gyr of evolution., 25 pages, 6 figures, 7 tables

We examined the O-C diagrams of eclipsing binary systems and selected these exhibiting cyclic shape, either sinusoidal or quasi sinusoidal. Assuming these variations being due to the Light Time Travel effect (LTE), we estimated the parameters of companions with the Monte Carlo method. As a result, we identified nearly two dozen of eclipsing systems that might have companions with a minimum mass larger than that of a neutron star. Their masses fall into the range between 1.7 and 34 solar masses. This sample of triples with high mass companions can be confirmed with the help of observations gathered by Gaia: parallaxes and astrometric measurements.

The four-color ($B$ $V$ $R_c$ $I_c$) light curves of V776 Cas are presented and analyzed using the Wilson-Devinney (W-D) method. It is discovered that V776 Cas is an early F-type (F2V) overcontact binary with a very high contact degree ($ f=64.6\,\%$) and an extremely low mass ratio ($q=0.130$), which indicate that it is at the final evolutionary stage of cool short-period binaries. The mass of the primary and secondary stars are calculated to be $M_1 = 1.55(\pm0.04)M_\odot$, $M_2 = 0.20(\pm0.01)M_\odot$. V776 Cas is supposed to be formed from an initially detached binary system via the loss of angular momentum due to the magnetic wind. The initial mass of the present primary and secondary components are calculated to be $M_{1i} = 0.86(\pm0.10)M_\odot$ and $M_{2i} = 2.13(\pm0.04)M_\odot$. The observed-calculated ($O$-$C$) curve exhibits a cyclic period variation, which is due to the light-travel time effect (LTTE) caused by the presence of a third component with a period of 23.7 years. The mass of the third component is estimated to be $M_3 = 1.04(\pm0.03)M_\odot$ and the orbital inclination of the third component is calculated to be $i' = 33.1^{\circ}$. The distance of the binary system to the mass center of the triple system is calculated to be $a'_{12} = 3.45AU$. The presence of the close-in tertiary component may play an important role in the formation and evolution of this binary system by drawing angular momentum from the central system., 17 pages, 4 figures. arXiv admin note: text overlap with arXiv:1608.00210, arXiv:1608.01042

II UMa is a late F-type (F5) contact binary with a close-in tertiary and a distant visual companion. According to the four-color ($B$ $V$ $R_c$ $I_c$) light curves' solutions of II UMa, it is a high fill-out (f=$86.6\,\%$) and low mass ratio ($q = 0.172$) contact binary system, which indicate that it is at the late evolutionary stage of late-type tidal-locked binary stars. The mass of the primary star and secondary one are calculated to be $M_1 = 1.99M_\odot$, $M_2 = 0.34M_\odot$. The primary star has evolved from ZAMS, but it is still before TAMS, and the secondary star is even more evolved. Considering the mass ratio ($M_3/M_1 = 0.67$) obtained by spectroscopic observations, the mass of the close-in tertiary is estimated to be $M_3 = 1.34M_\odot$. The period variations of the binary system is investigated for the first time. According to the observed-calculated ($O$-$C$) curve analysis, a continuous period increase at a rate of $dP/dt=4.88\times{10^{-7}}day\cdot year^{-1}$ is determined. It may be just a part of a cyclic period change, or the combinational period change of a parabolic variation and a cyclic one. More times of minimum light are needed to confirm this. The presence of the tertiary component may play an important role in the formation and evolution of this binary system by drawing angular momentum from the central system during the pre-contact stage., 12 pages, 4 figures. arXiv admin note: text overlap with arXiv:1608.00210

We present results of the modelling of multicolour light curves of 10 contact binary systems: V376 And, V523 Cas, CC Com, BX Dra, FG Hya, UZ Leo, XY Leo, AM Leo, EX Leo and RT LMi. The solutions resulted in a contact configuration for all systems. We found only FG Hya and UZ Leo 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 (BX Dra, AM Leo and RT LMi) show asymmetries and peculiarities in the observed light curves, interpreted as resulting from their magnetic activity.

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.

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 distributions of geometrical and physical parameters from the CCBS (Catalogue of Contact Binary Stars) and the ASAS-3 (The All Sky Automated Survey) are discussed. The distributions of orbital periods of light curves for 374 contact binaries from the CCBS as well as 3590 contact binaries, selected by Fourier decomposition of 4216 eclipsing binaries from the ASAS-3 database, are similar. The maxima of the period distributions are between 0.31– 0.40 days (0.25–0.32 days for W-type and 0.35–0.40 days for A-type) and 0.40–0.45 days for the CCBS and ASAS-3 dataset, respectively.

HS 0705+6700 is a short-period (P=2.3 hours), close binary containing a hot sdB-type primary and a fully convective secondary. We have monitored this eclipsing binary for more than 2 years and as a result, 32 times of light minimum were obtained. Based on our new eclipse times together with these compiled from the literature, it is discovered that the O-C curve of HS 0705+6700 shows a cyclic variation with a period of 7.15 years and a semiamplitude of 92.4 s. The periodic change was analyzed for the light-travel time effect that may be due to the presence of a tertiary companion. The mass of the third body is determined to be M3 sin i = 0.0377 (+/-0.0043) Msun when a total mass of 0.617 Msun for HS 0705+6700 is adopted. For orbital inclinations i >= 32.8, the mass of the tertiary component would be below the stable hydrogen-burning limit of M3~0.072 Msun, and thus it would be a brown dwarf. The third body is orbiting the sdB-type binary at a distance shorter than 3.6 astronomical units (AU). HS 0705+6700 was formed through the evolution of a common envelope after the primary becomes a red giant. The detection of a sub-stellar companion in HS 0705+6700 system at this distance from the binary could give some constraints on stellar evolution in such systems and the interactions between red giants and their companions., 10 pages, 3 figures To be published in Astrophysical Journal

Photometric solutions of two short period close binaries, V473 Cas and II Per, are derived with the 2003 version Wilson-Van Hamme code. It is shown that V473 Cas is a semidetached system with the primary component or the secondary component at its critical Roche Lobe, while II Per is a marginal contact binary system with both components filling their critical Roche Lobes, but with a large temperature difference between the two components. Both systems belong to near-contact binaries with the light curves enhanced around the left shoulder of secondary minimum, which can be explained by hot spots on the secondary components due to the mass transfer via a stream hitting the facing surface of the secondary components. The orbital period investigations based on all available times of light minimum show a secular period existing in these two targets decreasing at the rate of dP/dt = -7.61 x 10(-8) days yr(-1) for V473 Cas and dP/dt = -7.54 x 10(-8) days yr(-1) for II Per. The decrease of the orbital period can be the result of mass transfer from the primary component to the secondary one, which is consistent with the asymmetric light curves of V473 Cas and II Per. Therefore, V473 Cas and II Per are other examples displaying evidence of mass transfer between the two components. We have collected NCBs with secular period variation and find that almost all SD1-type NCBs show decreasing periods and enhancing luminosity on the left shoulder of secondary minimum in their light curves. So the semidetached configuration with the lobe filling primary is more plausible for V473 Cas. Furthermore, the detected enhancing amplitude of luminosity for SD2-type NCBs is smaller than other types of NCBs, which implies that the mass transfer from the primary component to the secondary one is stronger than the transfer in opposite direction. With the orbital period decrease, V473 Cas and II Per will evolve into overcontact binaries with true thermal contact.

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.

In this paper the collected times of minimum light of 1,140 eclipsing binary stars were statistically analyzed.

This paper presents the results of verification of known stars showing evidence of orbital eccentricity and apsidal motion.

This paper presents the results of the analysis of (O-C) diagrams of four eclipsing variables. The diagrams are based on times of minima collected in the Cracow database, which contains times of minima found in the literature, from observations at Mt. Suhora and Ulupinar Observatories, or determined using publicly-available photometric surveys (NSVS, ASAS etc).

A detailed analysis of an extensive collection of minima of Beta Lyrae was performed. After removing the effect of the orbital period increase, we applied the Fourier analysis to the O-C residuals. The most prominent periodicity is 283 days with amplitude of about 0.05 days. We confirm the period found by Van Hamme, Wilson & Guinan [1995, Astron. J., 110, 1350] from analysis of the photometric light curves of β Lyrae.

Investigations of times of minima in eclipsing binaries give us important clues concerning a number of quite different and frequently unrelated phenomena such as the apsidal motion, mass-loss, mass-transfer or gravitational radiation. It is hard to over-estimate the importance of these relatively easy observations. However, simple statistics show that much remains to be done to assure proper data for even a moderate--size sample of eclipsing binaries. Here we present essential results of such statistics made for stars brighter than 12 magnitude in minima (this limit assumed arbitrarily as accessible to small-telescope observers).