  Evidence of magnetic interaction between late-type stars and close-in giant
planets is provided by the observations of stellar hot spots rotating
synchronously with the planets and showing an enhancement of chromospheric and
X-ray fluxes. We investigate star-planet interaction in the framework of a
magnetic field model of a stellar corona, considering the interaction between
the coronal field and that of a planetary magnetosphere moving through the
corona. The energy budget of the star-planet interaction is discussed assuming
that the planet may trigger a release of the energy of the coronal field by
decreasing its relative helicity. The observed intermittent character of the
star-planet interaction is explained by a topological change of the stellar
coronal field, induced by a variation of its relative helicity. The model
predicts the formation of many prominence-like structures in the case of highly
active stars owing to the accumulation of matter evaporated from the planet
inside an azimuthal flux rope in the outer corona. Moreover, the model can
explain why stars accompanied by close-in planets have a higher X-ray
luminosity than those with distant planets. It predicts that the best
conditions to detect radio emission from the exoplanets and their host stars
are achieved when the field topology is characterized by field lines connected
to the surface of the star, leading to a chromospheric hot spot rotating
synchronously with the planet. The main predictions of the model can be
verified with present observational techniques, by a simultaneous monitoring of
the chromospheric flux and X-ray (or radio) emission, and spectropolarimetric
observations of the photospheric magnetic fields.
