PERIOD VARIATION AND SURFACE-ACTIVITY OF THE CONTACT BINARY VW CEPHEI

Long-term behavior of the well-known contact binary VW Cephei is discu ssed based on new multicolor (BV) photometry made over 8 years and hig h-resolution spectroscopy. The existence of an at least 7 year-long ac tivity cycle is confirmed using light curve disturbances as the indica tor of surface brightness variations. The orbital period of the system has been decreased further in accordance with previous predictions. I t is shown that the cyclic component of the period variation (O - C) c urve has an amplitude too large to be fully due to the light-time effe ct of the astrometric 3rd companion. It is suspected that the magnetic activity cycle of the primary component affects the orbital period si gnificantly. Simultaneous solution of light-and velocity curves (obtai ned during the ''quiet'' stage) resulted in new parameters of the syst em. The mass ratio (q = 0.35) is considerably larger than the previous spectroscopic value given by Hill (1989). High resolution optical spe ctroscopy made at the beginning of the new active cycle reveals H alph a emission arising from the chromosphere of the primary whereas the se condary lacks such emission. This fact gives further support to the th eoretical prediction that the primary component drives the activity of contact binaries. Fitting cool spots (Delta T-eff = 2000 K) to the su rface of the primary, the minimum spot coverage was found to be 12 % a t the active stage, while it is about 1 - 2 % at the quiet stage. An a nticorrelation of H alpha emission and spot position has been pointed out.