The Masses of Nearby Dwarfs and Brown Dwarfs with the HST
  The known nearby stars, moving in front of the background of distant stars and galaxies, create 'weak gravitational lensing' variations in their positions. These variations may be measurable with the HST, and they may allow a direct mass determination for the nearby stars. The cross section for the HST measurable astrometric effect is much larger than the cross section for the photometric effect which is measurable from the ground. The mass determination will be easier for the fainter nearby lenses which will be discovered in future searches of faint high proper motion stars. 
Bohdan Paczynski (Princeton University Observatory)