2016MNRAS.463.1342S

AM Her displays strong long-term activity with the high and low states. This investigation uses AAVSO optical data for a time series analysis of the long-term variations. Rapid changes of brightness (e.g. the orbital modulation) were smoothed out to emphasize the activity on super-orbital time-scale. I show that the character of this activity changed considerably on time-scales of years, which is reflected in a large evolution of the complicated histogram of the optical brightness. The high states are not the well-defined, narrow levels of brightness. I also show that AM Her displays transitions between the high and low states with the intermittently existing cycles. The longest uninterrupted series of transitions from the high to low state consists of seven episodes (about 6 yr). The existence of this series can be controlled by the lifetime of the active regions on the donor, which modulates the mass transfer rate. I show that the episodes of the high and low states accumulate in clusters, which produces an additional cycle after smoothing by the moving averages. The cycles of activity of the donor can explain this modulation. A single isolated short episode of the low state does not imply a break of this cycle. I also argue that the specific properties of star-spots and their migration caused by the differential rotation of the donor would be needed to explain the complex activity of AM Her.