On the mass-donating star in GRS 1915+105

We present an analysis of constraints on the companion mass for the Galacti c 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 al igned orbital axis, we constrain the ratio of the stellar radius to the bin ary 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, a pproximate black hole mass, and observations of the X-ray absorbing column density toward GRS 1915+105 rule out accretion via stellar wind from a mass ive companion, implying that accretion occurs via Roche lobe overflow. In t hat 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 si gnificant X-ray reprocessing in the infrared from GRS 1915+105 also provide s constraints on the mass and temperature range of a Roche lobe-filling mas s donor.