1998A&A...336L..25B

A new analysis of the published long-term photometric observations has revealed permanent active-longitude structures in four RS CVn stars: EI Eri, II Peg, σ Gem, and HR 7275. Two active longitudes separated by half of the period are found to dominate on the surface during all available seasons. The positions of the longitudes on three stars (EI Eri, II Peg, HR 7275) are migrating in the orbital reference frame, and there is no preferred orientation with respect to the line of centres in the binaries. The rate of migration is approximately constant. In case of σ Gem the active longitude migration is synchronized with the orbital motion in the direction of the line of centres in the binary. The active region lifetimes can be longer than the time span of the observations (≥15yr). The periods of the active longitude rotation are determined: for EI Eri 1.9510d, for II Peg 6.7066d, for σ Gem 19.604d, for HR 7275 28.263d. Long-term activity cycles of the stars are discovered from the analysis of the relative contribution of the two longitudes to the photometric variability. One longitude is found to be usually more active than the other at a given moment, and the change of the activity level between the longitudes is cyclic with periods of years. The switch of the activity takes a much shorter time, about a few months, similar to the ``flip-flop'' phenomenon found for FK Com stars. Moments of switching are regarded as new tracers of the activity, and total cycles, which return activity to the same longitude, are found to be for EI Eri 9.0yr, for II Peg 9.3yr, for σ Gem 14.9yr, for HR 7275 17.5yr.