The empirical mass-luminosity relation at Mv is presented for stars with masses 0.08-0.20M. based upon new observations made with Fine Guidance Sensor 3 on the Hubble Space Telescope. The targets are nearby, red dwarf multiple systems in which the magnitude differences are typically measured to ±0.1 mag or better. The Mv values are generated using the best available parallaxes and are also accurate to ±0.1 mag, because the errors in the magnitude differences are the dominant error source. In several cases this is the first time the observed sub-arcsecond multiples have been resolved at optical wavelengths. The mass-luminosity relation defined by these data reaches to Mv=18.5 and provides a powerful empirical test for discriminating the lowest mass stars from high-mass brown dwarfs at wavelengths shorter than 1 μm.
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