Stability of magnetic flux tubes in close binary stars

Photometric and Doppler imaging observations of active binaries indicate th e existence of starspots at preferred longitudes (position angles with resp ect to the companion star). We investigate the stability of magnetic flux t ubes in the convection zone of close, fast-rotating binary stars and explor e whether the observed preferred longitudes could be caused by tidal forces and the deformation of the active star. We assume a synchronized binary sy stem with spin axes perpendicular to the orbital plane and a rotation perio d of a few days. The tidal force and the deviation from spherical structure are considered in lowest-order perturbation theory. The magnetic field is in the form of toroidal magnetic flux rings, which are stored in mechanical equilibrium within the stably stratified overshoot region beneath the conv ection zone until the field has grown sufficiently strong for the undulator y instability to initiate the formation of rising loops. Frequencies and ge ometry of stable as well as growth rates of unstable eigenmodes are determi ned by linear stability analysis. Particular consideration is given to the question whether the effects of tidal forces and perturbations of the stell ar structure can force a rising flux loop to enter the convection zone at s pecific longitudes.