The magnetic activity cycle of II Pegasi: results from twenty-five years of wide-band photometry

We present an analysis of a sequence of light curves of the RS CVn-type bin ary II Pegasi extending from 1974 to 1998. The distribution of the spotted area versus longitude is derived by Maximum Entropy and Tikhonov regularize d maps, assuming a constant spot temperature (Lanza et al. 1998a). The spot pattern on the active K2 IV star can be subdivided into a component unifor mly distributed in longitude and a second unevenly distributed component, w hich is responsible for the observed photometric modulation. The uniformly distributed component appears to be possibly modulated with an activity cyc le of similar to 13.5 yr. The unevenly distributed component is mainly conc entrated around three major active longitudes. The spot activity appears pr actically permanent at one longitude, but the spot area changes with a cycl e of similar to 9.5 yr. On the contrary, the spot activity is discontinuous at the other two longitudes, and it switches back and forth between them w ith a cycle of similar to 6.8 yr. However, before each switching is complet ed, a transition phase of similar to 1.05 yr, during which both longitudes are active, occurs. After this transient phase, spot activity remains local ized at one of the two longitudes for similar to 4.7 yr untill another swit ching event occurs, which re-establishes spot activity at the other longitu de. The longitude separation between the permanent and the switching active longitudes is closest during the switching phases and it varies along the similar to 6.8 yr cycle. Different time scales characterize the activity at the permanent longitude and at the switching longitudes: a period of simil ar to 9.5 yr is related to the activity cycle at the permanent longitude, a nd a period of similar to 4.3 yr characterizes the spot life time at the sw itching longitudes in between switching events. The photometric period of the active star changes from season to season wit h a relative amplitude of 1.5% and a period of similar to 4.7 yr. Such a va riation of the photometric period may be likely associated with the phase s hift of the light curves produced by the switching of spot activity from on e active longitude to the other. The permanently active longitude shows a s teady migration towards decreasing orbital phases, with an oscillating migr ation rate along the 9.5 yr cycle period and nearly in phase with the varia tion of its spotted area. The amplitude of the differential rotation derive d from such a behaviour is of the order of similar to 0.023%, about one ord er of magnitude smaller than estimated by Henry et al. (1995). The other tw o active longitudes migrates also towards decreasing orbital phase, but at a discontinuous rate. There appears to be no correlation between the locati on of the active longitudes with respect to the line joining the two compon ents of the system and their activity level.