Gravitational quadrupole-moment variations in active binaries

The principal characteristics of the observed orbital period variation in magnetically active close binaries are briefly reviewed and the theoretical models proposed to interpret them are presented. In particular, we focus on the models proposed by Applegate (1992) and Lanza, Rodonò & Rosner (1998a) to explain the short-term modulation of the orbital period, as a consequence of the changes of the gravitational quadrupole moment of the active component driven by a cyclic hydromagnetic dynamo. Recent observational results support our interpretation and the constraints on the intensity of the internal magnetic fields that are required by the proposed mechanism, are discussed. A novel, stringent test of our model based on future asteroseismic space observations of oscillation mode splitting is also briefly presented.