The mass of the black hole in v 404 Cygni

We have obtained a K-band infrared light curve of the low-mass X-ray binary transient V404 Cygni. We model it as the ellipsoidal variation of a secondary star distorted in the gravitational field of a black hole, and thus deduce the inclination of the binary. Combining this with previously published spectroscopic data, we determine the mass of the black hole to be approximately 12 MΘ © Royal Astronomical Society.