Polarization in binary microlensing events
  The light received by source stars in microlensing events may be
significantly polarized if both an efficient photon scattering mechanism is
active in the source stellar atmosphere and a differential magnification is
therein induced by the lensing system. The best candidate events for observing
polarization are highly magnified events with source stars belonging to the
class of cool, giant stars {in which the stellar light is polarized by photon
scattering on dust grains contained in their envelopes. The presence in the
stellar atmosphere of an internal cavity devoid of dust produces polarization
profiles with a two peaks structure. Hence, the time interval between them
gives an important observable quantity directly related to the size of the
internal cavity and to the model parameters of the lens system.} We show that
{during a microlensing event} the expected polarization variability can solve
an ambiguity, that arises in some cases, related to the binary or planetary
lensing interpretation of the perturbations observed near the maximum of the
event light-curve. We consider a specific event case for which the parameter
values corresponding to the two solutions are given. Then, assuming a
polarization model for the source star, we compute the two expected
polarization profiles. The position of the two peaks appearing in the
polarization curves and the characteristic time interval between them allow us
to distinguish between the binary and planetary lens solutions.
