Surface reconstructions of active close binary stars based on photometric and
spectroscopic observations reveal non-uniform starspot distributions, which
indicate the existence of preferred spot longitudes (with respect to the
companion star). We consider the equilibrium and linear stability of toroidal
magnetic flux tubes in close binaries to examine whether tidal effects are
capable to initiate the formation of rising flux loops at preferred longitudes
near the bottom of the stellar convection zone. The tidal force and the
deviation of the stellar structure from spherical symmetry are treated in
lowest-order perturbation theory assuming synchronised close binaries with
orbital periods of a few days. The frequency, growth time, and spatial
structure of linear eigenmodes are determined by a stability analysis. We find
that, despite their small magnitude, tidal effects can lead to a considerable
longitudinal asymmetry in the formation probability of flux loops, since the
breaking of the axial symmetry due to the presence of the companion star is
reinforced by the sensitive dependence of the stability properties on the
stellar stratification and by resonance effects. The orientation of preferred
longitudes of loop formation depends on the equilibrium configuration and the
wave number of the dominating eigenmode. The change of the growth times of
unstable modes with respect to the case of a single star is very small.Comment: 11 pages, 11 figures, accepted for publication in A&