The authors report the first infrared light curve of GS 2000+25. By modelling the brightness variations as ellipsoidal modulations of the secondary star, the authors find the mass of the compact object to lie between 4.8 and 14.4 Msun (90 per cent confidence) for secondary masses ≤0.7 Msun; 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 the authors find the inclination of the system to lie between 43° and 69°.