THE MASSES OF NEARBY DWARFS AND BROWN DWARFS WITH THE HST

The known nearby stars, moving in front of the background of distant s tars 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 fainte r nearby lenses which will be discovered in future searches of faint h igh proper motion stars.