The rotational broadening and the mass of the donor star of GRS 1915+105

The binary parameters of the microquasar GRS 1915+105 have been determined by the detection of Doppler-shifted 12CO and 13CO lines in its K-band spectrum (Greiner et al. [CITE]). Here, we present further analysis of the same K-band VLT spectra and we derive a rotational broadening of the donor star of km s-1 from the 12CO/13CO lines. Assuming that the K-type star is tidally locked to the black hole and is filling its Roche-lobe surface, then the implied mass ratio is . This result, combined with () = (33.5 d, 140 km s-1, 66°) gives a more refined mass estimate for the black hole, , than previously estimated, using an inclination of (Fender et al. [CITE]) as derived from the orientation of the radio jets and a more accurate distance. The mass for the early K-type giant star is , consistent with a more evolved stripped-giant donor star in GRS 1915+105 than, for example, the donor star of the prototype black-hole X-ray transient, V404 Cyg which has the longest binary period after GRS 1915+105.