Separate Spectra of Stars of Known Very Low Mass in Nearby Close Binaries

The Hubble Space Telescope has recently demonstrated its ability in astrometry to separate and measure with high precision the relative orbital position of faint, nearby, low- mass binaries which are barely resolvable by ground based techniques. Since the servicing mission it has the capability of making an even greater advance over ground-based spectroscopy by acquiring separate spectra of the members of faint, close binaries, the only ones where mass can be determined. It is impossible to get these spectra from the ground because the separations of these binaries are too small to be isolated at the spectrograph slit. Knowledge of stellar masses at the ``brown dwarf'' boundary is important in many interrelated areas such as the end of the stellar mass function, theory of stellar evolution,end of the main sequence, and the galactic mass, age and evolution. Currently the objects of low stellar and sub-stellar mass are strong candidates as a source of ``missing'' galactic mass, but a means of estimating the masses of the many known single, low-luminosity and late-type M-dwarf objects is needed. Good separate spectra of these low-mass objects are needed for advancement of the mass calibration of very late-type dwarfs in order to extend the mass function lower.