1998PASP..110..425C

The Hubble Space Telescope Faint Object Spectrograph has been used to acquire separate spectra of each of the component stars in the binaries Ross 614A,B (Gl 234A,B) and L722-22A,B (GJ 1005). Spectral types have been determined for each: M4.0 V, M5.5 V, M3.5 V, and M5.0 V, respectively, relative to a series of ground-based CCD spectra, in the 6500-8000 Å region. The four members of the two binaries form a narrow locus in the spectral type-mass plane, from 0.18 to 0.08 M_☉ and reaching 0.08 M_☉ near spectral type M6.0. Both binaries are believed to be members of a relatively young disk population. Their masses and newly determined spectra imply that the apparently single M dwarf stars, of a similar population and with spectral types of M6, are likely to be near, or less massive than, 0.08 M_☉. Presumably, dwarf M7, M8, and M9 stars of a similar age and population are in a declining mass sequence and thus should reach well below the theoretical main-sequence mass limit of 0.08 M_☉ and by current theory are substellar or ``brown dwarf'' objects.