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, 1992; Lanza et al., 1998b
). 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, 2002; Donati
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., 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.
