Multiple and changing cycles of active stars

Aims. We study the time variations in the cycles of 20 active stars based on decade-long photometric or spectroscopic observations.Methods. A method of time-frequency analysis, as discussed in a companion paper, is applied to the data.Results. Fifteen stars definitely show multiple cycles, but the records of the rest are too short to verify a timescale for a second cycle. The cycles typically show systematic changes. For three stars, we found two cycles in each of them that are not harmonics and vary in parallel, indicating a common physical mechanism arising from a dynamo construct. The positive relation between the rotational and cycle periods is confirmed for the inhomogeneous set of active stars.Conclusions. Stellar activity cycles are generally multiple and variable.