Time scales of activity cycles in solar-type stars and K-M dwarfs

Cycles of activity similar to the well-known spots cycle on the Sun are known now uxes of H&K Ca IIrs and K-M dwarfs. A study of the intensity of fl lines in the HK project or the behavior of brightness on long-term spans allow us to find cyclic changes of activity on many objects. We collected the published data about stellar cycles and obtained our own new photometric results for 9 active G-M dwarfs using photometric databases and considered the dominant detected or suspected cycles for the sample of 65 solar-type stars and red dwarfs. About 50% of stars have activity cycles comparable with the solar 11-year cycle (7-18 yr). This group includes stars with periods of rotation >25 d. The faster rotating G-K dwarfs (15%) as the more active show longer cycles up to 40 yr. Several of K and M rapid rotating dwarfs (10%) have the longest time scale of variations in a yearly mean brightness up to 80 years. The investigation of long-term cycles of the stellar activity is limited by data series, only a few of them cover a time span of about 100 years. So, at present we can study cycles of activity less than 50 years, longer cycles can only be suspected.