The time scale for such a modulation is of several decades and its cyclic origin is still an assumption. The relationship between the orbital period modulation and the starspot cycle is not yet clear. In some stars the spot cycle is half as long as the orbital period modulation (Keskin et al., 1994; Rodonò et al., 1995; Lanza et al., 1998a), in others these two cycles are approximately of the same length (Ibanoglu et al., 1994; Hall, 1991b). On the other hand, the length of the cycle of the orbital period modulation seems to be correlated with the angular velocity of the star. This supports the suggestion that a distributed non-linear dynamo is at work in the convection envelopes of very active stars and that it strongly affects the differential rotation (Lanza and Rodonò, 1999b). If the cyclic origin of the orbital period modulation and its relation to the spot cycle is confirmed, this can be used as a proxy for magnetic cycles in Algols and cataclysmic variables where photometric modulation of the cool secondary is difficult to detect.
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