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

Optical coronagraph images of the high proper motion astrometric binar y 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 G l 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 demo nstrates the usefulness of optical coronagraphy for identifying very l ow mass objects.