Dynamical Mass of GJ 802B: A Brown Dwarf in a Triple System

We report a dynamical measurement of the mass of the brown dwarf GJ 802B using aperture-masking interferometry and astrometry. In addition, we report the discovery that GJ 802A is itself a close spectroscopic noneclipsing binary with a 19 hr period. We find the mass of GJ 802B to be 0.063 +/- 0.005 M_⊙. GJ 802 has kinematics inconsistent with a young star and more consistent with the thick-disk population, implying a system age of ~10 Gyr. However, model evolutionary tracks for GJ 802B predict system ages of ~2 Gyr, suggesting that brown dwarf evolutionary models may be underestimating luminosity for old brown dwarfs.