The mass, luminosity and mass-loss rate of the donor of the V1487 Aql/GRS 1915+105 binary system

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.