DETECTION OF A VERY-LOW MASS COMPANION TO THE ASTROMETRIC BINARY GLIESE 105A

Optical coronagraph images of the high proper motion astrometric binary G1 105A reveal a very red companion, Gl 105C, located 3.''27 from Gl 105A at P.A. 287 degrees. At this location, it is not clear whether Gl 105C can fully account for the astrometric perturbation of Gl 105A. Aperture photometry gives I-c = 12.6 and R(c) - I-c = 3.7 for Gl 105C, indicating that it is a very low mass M dwarf. Using the observed I-c, an empirical M(1) versus I-K relation, and an assumed distance of 8.2 pc to Gl 105A, M(K) = 9.7 is derived for Gl 105C. An empirical mass-M, relation for low-mass stars suggests a mass of 0.084 M. for Gl 105C, which is just above the minimum mass for stable hydrogen burning. G1 105C was not detected in previous K-band searches; its detection demonstrates the usefulness of optical coronagraphy for identifying very low mass objects.