  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-K2 correlation to the disc Halpha emission line detected in Gran
Telescopio Canarias spectra and obtain K2=521+-52 km/s. This leads to a mass
function f(M)=4.1+-1.2 Msun, thus indicating the presence of a black hole in
this historic X-ray transient. Furthermore, from the depth of the Halpha trough
and the quiescent light curve we constrain the binary inclination to i=61+-15
deg, while the detection of superhumps sets an upper limit to the donor to
compact star mass ratio q=M2/M1<=0.25. Our de-reddened (r-i) colour is
consistent with a ~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
M1=6.4+3.2-2.0 Msun at 68 per cent confidence. We also constrain the distance
to GRS 1716-249 to 6.9+-1.1 kpc, placing the binary ~0.8 kpc above the Galactic
Plane, in support of a large natal kick.

High Energy Astrophysical Phenomena;Solar and Stellar Astrophysics