Precision Masses of the Low - Mass Binary System Gj 623

We have used Aperture Masking Interferometry and Adaptive Optics (AO) at the Palomar 200” to obtain precise mass measurements of the binary M dwarf GJ 623. AO observations spread over 3 years combined with a decade of radial velocity measurements constrain all orbital parameters of the GJ 623 binary system accurately enough to critically challenge the models. The dynamical masses measured are m1 = 0.360±0.014M⊙ (4%) and m2 = 0.120±0.0023M⊙ (2%) for the primary and the secondary respectively. Models are not consistent with color and mass, requiring very low metallicities. Subject headings: binary, stars: luminosity functions, mass functions, techniques: AO, interferometric