Precise masses for Wolf 1062 AB from Hubble Space Telescope interferometric astrometry and McDonald Observatory radial velocities

We present an analysis of astrometric data from Fine Guidance Sensor 3 (FGS 3), a white-light interferometer on HST, and of radial velocity data from two ground-based campaigns. We model the astrometric and radial velocity measurements simultaneously to obtain parallax, proper motion, and component masses for Wolf 1062 (G1 748; M3.5 V). To derive the mass fraction, we relate FGS 3 fringe scanning observations of the science target to a reference frame provided by fringe tracking observations of a surrounding star field. We obtain an absolute parallax (pi (abs) = 98.0 +/- 0.4 mas) yielding M (A) = 0.379 +/- 0.005 M. and M (B) = 0.192 +/- 0.003 M ., high-quality comonent masses with errors of only 1.5%.