  We re-consider the polarization of the star light that may arise during
microlensing events due to the high gradient of magnification across the
atmosphere of the source star, by exploring the full range of microlensing and
stellar physical parameters. Since it is already known that only cool evolved
giant stars give rise to the highest polarization signals, we follow the model
by Simmons et al. (2002) to compute the polarization as due to the photon
scattering on dust grains in the stellar wind. Motivated by the possibility to
perform a polarization measurement during an ongoing microlensing event, we
consider the recently reported event catalog by the OGLE collaboration covering
the 2001-2009 campaigns (OGLE-III events), that makes available the largest and
more comprehensive set of single lens microlensing events towards the Galactic
bulge. The study of these events, integrated by a Monte Carlo analysis, allows
us to estimate the expected polarization profiles and to predict for which
source stars and at which time is most convenient to perform a polarization
measurement in an ongoing event. We find that about two dozens of OGLE-III
events (about 1 percent of the total) have maximum polarization degree in the
range 0.1 < P_{\rm max} <1 percent, corresponding to source stars with apparent
magnitude I < 14.5, being very cool red giants.This signal is measurable by
using the FORS2 polarimeter at VLT telescope with about 1 hour integration
time.
