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 greater than or similar to 3 M-.) 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 semimajor axis to be R-s/a < 0.342, based on the lack of X-ray eclipses. We then show that these constraints together with the X-ray luminosity, approximate black hole mass, and observations of the X-ray absorbing column density toward GRS 1915+105 rule out accretion via stellar wind from a massive companion, implying that accretion occurs via Roche lobe overflow. In that case, the constraint on R-s/a rules out any companion with M-s > 19.4 M-. for a black hole mass of M-BH similar or equal to 30 M-.. 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.