Long-term photometric study of FK Comae Berenices and HD 199178

Abstract

Aims. The rapidly rotating active stars FK Com and HD 199178 are investigated using extensive long-term photometry. The data set includes previously unpublished photometry, obtained at the Rozhen National Astronomical Observatory. We look for periodicities in the long-term photometric behaviour of both stars, as well as for spots, using light curve modelling. Special attention is paid to the behaviour of the "flip-flop" phenomenon. Methods. Light curves in UB V of FK Com and HD 199178 are used to construct an O-C diagram for each star, and they are modelled to obtain spot distribution. Results. The main result for FK Com is that the "flip-flop" phenomenon is a superposition of an oscillating process of activity and real and fast "flip-flops". For the oscillating component, we obtained a cycle of 5.8 years. The same cycle is also present in other photometric characteristics. The O-C diagram for HD 199178 shows a cycle of 4.2 years. Modelling of the light curves of both stars reveals a large high-latitude spot on each star and also smaller spots in the equatorial belt or at intermediate latitudes. © ESO 2007.