  The donor in the microquasar GRS 1915+105 is a low-mass giant. Such a star
consists of a degenerate helium core and a hydrogen-rich envelope. Both
components are separated by a hydrogen burning shell. The structure of such an
object is relatively simple and easy to model. Making use of the observational
constraints on the luminosity and the radius of the donor, we constrain the
mass of this star with evolutionary models. We find a very good agreement
between the constraints from those models and from the observed rotational
broadening and the NIR magnitude. Combining the constraints, we find solutions
with stripped giants of the mass of $\geq\!0.28{\rm M}_{\odot}$ and of the
spectral class K5 III, independent of the distance to the system, and a
distance-dependent upper limit, $\lesssim\!1{\rm M}_{\odot}$. We also calculate
the average mass transfer rate and the duty cycle of the system as a function
of the donor mass. This rate is much below the critical rate (at which the
system would become persistent), and the duty cycle is less than 20 per cent.
