  We introduce a new model to explain the modulation of the orbital period
observed in close stellar binary systems based on an angular momentum exchange
between the spin of the active component and the orbital motion. This
spin-orbit coupling is not due to tides, but is produced by a non-axisymmetric
component of the gravitational quadrupole moment of the active star due to a
persistent non-axisymmetric internal magnetic field. The proposed mechanism
easily satisfies all the energy constraints having an energy budget about
100-1000 times smaller than those of previously proposed models and is
supported by the observations of persistent active longitudes in the active
components of close binary systems. We present preliminary applications to
three well-studied binary systems to illustrate the model. The case of stars
with hot Jupiters is also discussed showing that no significant orbital period
modulation is generally expected on the basis of the proposed model.
