  Microlensing consists in two major effects: (1) variation in the apparent
position of the background sources (astrometric component) and (2) flux
variations of the background sources (photometric component). While the latter
has been extensively used in the search for dark objects in the Galactic disk
and halo (projects like MACHO (Alcock et al, 1997), EROS (Derue et al, 1999),
OGLE (Paczynski et al, 1994)), the first effect has not yet been part of a
systematic observational program, simply because the observations of very
slight displacements in the positions of background sources requires an
astrometric accuracy which current telecopes do not yet provide. We investigate
here whether the astrometric accuracy of GAIA could enable such measurements
and, as a consequence, enable new, direct and original measurements of the mass
of nearby stars.
