  Multiple-epoch, multicolor images of the astrometric binary Gliese 105A and
its very low mass companion Gliese 105C have been obtained using the Hubble
Space Telescope's Wide Field Planetary Camera 2 (WFPC2) and Near-Infrared
Camera and Multi-Object Spectrometer (NICMOS). The optical and near-infrared
colors of Gl 105C strongly suggest a spectral type of M7V for that star.
Relative astrometric measurements spanning 3 yr reveal the first evidence of Gl
105C's orbital motion. Previous long-term astrometric studies at Sproul and
McCormick Observatories have shown that the period of Gl 105A's perturbation is
approximately 60 yr. To satisfy both the observed orbital motion and Gl 105A's
astrometric period, Gl 105C's orbit must have an eccentricity of approximately
0.75 and a semimajor axis of approximately 15 AU. Measurements of Gl 105A's
radial velocity over 12 yr show a linear trend with a slope of 11.3 m/s/yr,
which is consistent with these orbital constraints and a nearly face-on orbit.
As no other faint companions to Gl 105A have been detected, we conclude that Gl
105C is probably the source of the 60-yr astrometric perturbation.
