The history and source of mass-transfer variations in AM Herculis

The optical brightness of magnetic cataclysmic variables without accretion discs is a direct measure of the near-instantaneous mass-transfer rates fro m the late-type secondary stars to the magnetic white dwarfs in these semi- detached binary systems. We derive the mass-transfer history of the magneti c cataclysmic variable AIM Herculis from its long-term visual light curve a nd from bolometric corrections obtained from a number of X-ray observations covering various accretion states. On average. less than 25% of the maximu m observed mass-transfer rate occurs. Assuming that the mass-transfer rate is modulated by stellar starspots on the secondary stars, we convert the de rived mass-transfer rates into spot filling factors at the L-1-point. A sta tistical model for the coverage fraction and size distribution of random sp ots near the L-1-point in AM Her suggests that the spot filling factor is r oughly 0.5 for a fitted power-law law distribution of starspot radii, i.e. about half the surface of the star near the L-1-point is covered with spots . This density can only be explained if the spottedness of the L-1-point is unusual - for instance if spot groups are forced to wander towards the L-1 -point - or if a large-scale magnetic spot group produced by an alpha (2)-d ynamo slowly drifts in and out of the L-1-region. The former solution predi cts that the occurrence of long-term high- and low-states is random and the latter that the long-term light curves of polars are quasi-periodic; the l ight curve of AM Her may suggest periods of order a decade. Finally, we dis cuss the relevance of this result to the mass-transfer variations of other cataclysmic variables.