The contact binary VW Cephei revisited: surface activity and period variation

Abstract

Context. Despite the fact that VW Cephei is one of the most well-studied contact binaries in the literature, there is no fully consistent model available that can explain every observed property of this system. Aims. Our aims are to obtain new spectra along with photometric measurements, to analyze what kind of changes may have happened in the system in the past two decades, and to propose new ideas for explaining them. Methods. For the period analysis we determined ten new times of minima from our light curves, and constructed a new O–C diagram of the system. Radial velocities of the components were determined using the cross-correlation technique. The light curves and radial velocities were modeled simultaneously with the PHOEBE code. All observed spectra were compared to synthetic spectra and equivalent widths (EWs) of the Hα line were measured on their differences. Results. We re-determine the physical parameters of the system according to our new light curve and spectral models. We confirm that the primary component is more active than the secondary, and there is a correlation between spottedness and the chromospheric activity. We propose that the flip-flop phenomenon occurring on the primary component could be a possible explanation of the observed nature of the activity. To explain the period variation of VW Cep, we test two previously suggested scenarios: the presence of a fourth body in the system, and the Applegate-mechanism caused by periodic magnetic activity. We conclude that although none of these mechanisms can be ruled out entirely, the available data suggest that mass transfer with a slowly decreasing rate provides the most likely explanation for the period variation of VW Cep.