A mechanism for orbital period modulation in close binaries

Some eclipsing variables are observed to undergo orbital period modulations of amplitude 10 exp -5 over time scales of decades or longer. These modulations can be explained by the gravitational coupling of the orbit to variations in the shape of a magnetically active star in the system. The variable deformation of the active star is produced by variations in the distribution of angular momentum as the star goes through its activity cycle. The torque needed to redistribute the angular momentum can be exerted by a mean subsurface magnetic field of several kilogauss.