On the Mass-Donating Star in GRS 1915+105

We present an analysis of constraints on the companion mass for the Galactic microquasar GRS 1915+105. Using the known inclination angle and stability of the jet axis, we can rule out massive ($M_s ga 3 M_{sun}$) companions with orbital axes not aligned with the jet axis. For an aligned orbital axis, we constrain the ratio of the stellar radius to the binary semi-major axis to be $R_s/a 19.4 M_{sun}$ for a black hole mass of $M_{BH} simeq 30 M_{sun}$. The lack of significant X-ray reprocessing in the infrared from GRS 1915+105 also provides constraints on the mass and temperature range of a Roche-lobe-filling mass donor.Comment: 18 pages, including 2 figures; to appear in ApJ Letter