  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., 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 V sin i=26+-3 km/s 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 q = M_d/M_x = 0.058+-0.033. This result,
combined with (P, K, i)=(33.5 d, 140 km/s, 66 deg) gives a more refined mass
estimate for the black hole, $M_x=14.0+-4.4 M_{\odot}$, than previously
estimated, using an inclination of i=66+-2 deg (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 $M_d=0.81\pm0.53 M_{\odot}$, 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.
