  Observations of magnetically active close binaries with orbital periods of a
few days reveal the existence of starspots at preferred longitudes (with
respect to the direction of the companion star). We numerically investigate the
non-linear dynamics and evolution of magnetic flux tubes in the convection
zoneof a fast-rotating component of a close binary system and explore whether
the tidal effects are able to generate non-uniformities in the surface
distribution of erupting flux tubes. Assuming a synchronised system with a
rotation period of two days and consisting of two solar-type components, both
the tidal force and the deviation of the stellar structure from spherical shape
are considered in lowest-order perturbation theory. The magnetic field is
initially stored in the form of toroidal magnetic flux rings within the stably
stratified overshoot region beneath the convection zone. Once the field has
grown sufficiently strong, instabilities initiate the formation of rising flux
loops, which rise through the convection zone and emerge at the stellar
surface. We find that although the magnitude of tidal effects is rather small,
they nevertheless lead to the formation of clusters of flux tube eruptions at
preferred longitudes on opposite sides of the star, which result from the
cumulative and resonant character of the action of tidal effects on rising flux
tubes. The longitude distribution of the clusters depends on the initial
parameters of flux tubes in the overshoot region like magnetic field strength
and latitude, implying that there is no globally unique preferred longitude
along a fixed direction.
