  Gravitational microlensing, in case of relevant finite source size effects,
provides an unique tool for the study of stellar atmospheres through the
enhancement of a characteristic polarization signal. Here, we consider a set of
highly magnified events and show that for different types of source stars (as
hot, late type main sequence and cool giants) showing that the polarization
strength may be of $\simeq 0.04$ percent for late type stars and up to a few
percent for cool giants.
