1994PASP..106.1075R

Recent interest in long periods which may be present in cataclysmic variables has been motivated principally by the idea that solar-like magnetic cycles might be important. For the brighter members of the class, light curves compiled from amateur visual observations are an idea resource for testing the presnece of such periods. Because of the sheer immensity of the data, and the relative constancy of the average human eye over decades, the detection limits are more sensitve than could be achieved by any feasible photographic or photoelectric means. Here we present results for four well-studied stars, and evaluate the case for periods in other CVs. The conclusion from all of this is that _none of the stars studied to date exhibits a good case for strictly periodic variability, and it is not even clear whether there is a preferred timescale._ Nevertheless, the observed amplitudes (0.2 mag) and _apparent timescales (5-40 yrs) of variability are plausible consequences from solar-like magnetic cycles.

We propose that the observed light variations on decade timescales are produced by the same mechanism which underlies the decade-timescale orbital period changes seen in eclipsing binaries. In particular, we extend a theory of P_orb changes to the case of accretion-powered binaries, and show that it predicts accretion rate variations of amplitude delta-M/M ~= 0.1, consistent with observation. The proximate cause of the P_orb and M variations is the cyclic transfer of angular momentum to and from the outer layers of the contact star. The underlying cause of these phenomena is solar-type cycles of the contact star.