The connection between orbital period variation and magnetic activity cyclic behaviour in close binaries with late-type components is addressed by discussing recent observational studies of Algols, RS CVn's, W UMa's and CVs. A theoretical model based on the Applegate's mechanism seems capable of explaining the observed orbital period modulation in terms of cyclic changes of a gravitational quadrupole moment induced by a magnetic activity cycle affecting one of the binary components. In such a case, the study of orbital period modulations offers a promising tool to investigate hydromagnetic dynamos operating in the interior of active stars, in particular, to address the fundamental question of the interaction between rotation and magnetic fields in nonlinear dynamo regimes. Moreover, interesting applications to planetary systems with a magnetically active central star are discussed.