6.3 Orbital period modulation

A modulation of the orbital period in close binaries with cool active components is a phenomenon which is suggested to be associated with magnetic activity cycles (Hall, 1989). It can be induced by changes of the angular momentum and magnetic field distribution within the convection envelope of the active component, which may occur during an activity cycle (Applegate, 1992Lanza et al., 1998bJump To The Next Citation Point). The variation of the gravitational quadrupole momentum induces changes of the gravitational acceleration of the companion star, leading to the observed modulation of the orbital period. Possible variations of the stellar differential rotation may also be a manifestation of this mechanism (Collier Cameron and Donati, 2002Donati et al., 2003a).

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., 1994Rodonò et al., 1995Lanza et al., 1998a), in others these two cycles are approximately of the same length (Ibanoglu et al., 1994Hall, 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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