The orbital period, black hole mass, and distance to the X-ray transient GRS 1716-249 ( =N Oph 93)

We present evidence for a 0.278(8) d ( = 6.7 h) orbital period in the X-ray transient GRS 1716-249 ( = N Oph 93), based on a superhump modulation detected during the 1995 mini-outburst plus ellipsoidal variability in quiescence. With a quiescent magnitude of r = 23.19 +/- 0.15 N Oph 93 is too faint to warrant a full dynamical study through dedicated time-resolved spectroscopy. Instead, we apply the FWHM-K-2 correlation to the disc H alpha emission line detected in Gran Telescopio Canarias spectra and obtain K-2 = 521 +/- 52 km s( -1). This leads to a mass function f(M) = 4.1 +/- 1.2 M-circle dot, thus indicating the presence of a black hole in this historic X-ray transient. Furthermore, from the depth of the H alpha trough and the quiescent light curve we constrain the binary inclination to i = 61 +/- 15 degrees, while the detection of superhumps sets an upper limit to the donor to compact star mass ratio q = M-2/M-1 less than or similar to 0.25. Our de-reddened ( r - i) colour is consistent with a approximate to K6 main sequence star that fills its Roche lobe in a 0.278 d orbit. Using all this information we derive a compact object mass M-1 = 6.4(-2.0)(+3.2) M-circle dot at 68 per cent confidence. We also constrain the distance to GRS 1716-249 to 6.9 +/- 1.1 kpc, placing the binary similar to 0.8 kpc abo v e the Galactic Plane, in support of a large natal kick.