A 6.5-day periodicity in the recurrent nova V404 Cygni implying the presence of a black hole

THE X-ray transient source GS2023 + 338 was discovered in out-burst by the Ginga satellite in 1989 (ref. 1) and has since been identified with the previously known recurrent nova V404 Cygni2. This system is recognized to be a low-mass X-ray binary3, with X-ray behaviour similar to black hole systems4, but attempts to deduce an orbital period from photometry5-9 and spectroscopy10,11 have yielded modulations with periods from 10 minutes to 6 hours. Two years after the outburst, we have used the William Herschel Telescope to find absorption features in V404 Cyg characteristic of a late G or early K star with a radial velocity curve of amplitude 211 ± 4km s-1 and period 6.473 ± 0.001 days. The deduced mass function of 6.26±0.31 M⊙ is a firm lower limit to the mass of the compact object, which for reasonable assumptions of orbital inclination and companion star mass must be a black hole with probable mass in the range 8-15.5 M⊙. We consider this the most persuasive case yet for the existence of a black hole. © 1992 Nature Publishing Group.