The mass of the black hole in GS2000+25 (=QZ Vul)

We report the first infrared light curve of GS 2000 + 25. By modelling the brightness variations as ellipsoidal modulations of the secondary star, we find the mass of the compact object to lie between 4.8 and 14.4 M. (90 per cent confidence) for secondary masses less than or equal to 0.7 M.; this result is independent of the mass ratio, q. The upper inclination limit of the system is more dependent on q, although the lower inclination limit is almost independent of it, and for q = 20 we find the inclination of the system to lie between 43 degrees and 69 degrees.