  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 $m_{1}=0.371\pm0.015 M_{\sun}$ (4%) and $m_{2}=0.115\pm0.0023
M_{\sun}$ (2%) for the primary and the secondary respectively. Models are not
consistent with color and mass, requiring very low metallicities.
