The binary parameters of the microquasar GRS 1915+105 have been determined by the detection of Doppler-shifted ^ { 12 } CO and ^ { 13 } CO lines in its K-band spectrum ( Greiner et al. , 2001 , Nature , 414 , 522 ) .
Here , we present further analysis of the same K-band VLT spectra and we derive a rotational broadening of the donor star of \upsilon \sin i = 26 \pm 3 km s ^ { -1 } from the ^ { 12 } CO/ ^ { 13 } CO 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 q = \frac { M _ { d } } { M _ { x } } = 0.058 \pm 0.033 .
This result , combined with ( P,K,i ) = ( 33.5 d , 140 km s ^ { -1 } , 66 ^ { \circ } ) gives a more refined mass estimate for the black hole , M _ { x } = 14.0 \pm 4.4 ~ { } M _ { \odot } , than previously estimated , using an inclination of i = 66 ^ { \circ } \pm 2 ^ { \circ } ( Fender et al .
1999 ) as derived from the orientation of the radio jets and a more accurate distance .
The mass for the early K-type giant star is M _ { d } = 0.81 \pm 0.53 ~ { } M _ { \odot } , 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 .