  We present the first radial velocity curve of the companion star to BW Cir
which demonstrates the presence of a black hole in this X-ray transient which
recorded outbursts in 1987 and 1997 (and possibly 1971-2). We identify weak
absorption features corresponding to a G0-5III donor star, strongly veiled by a
residual accretion disc which contributes 61-65 % of the total light at lambda
6300. The Doppler motions of these features trace an orbit of P=2.54448 days
(or its 1-yr alias of P=2.56358 days) and velocity semi-amplitude K2=279 +- 5
km/s (or K2=292 +- 5 km/s). Both solutions are equally possible. The mass
function implied by the shorter period solution is f(M)= 5.75 +- 0.30 Msun
which, combined with the rotational broadening of the tidally locked companion
(V sin i =71 +- 4 km/s), yields a compact object mass of M1 sin3 i = 7.34 +-
0.46 Msun. This is substantially above the mass of a neutron star under any
standard equation of state of nuclear matter. The companion star is probably a
G subgiant which has evolved off the main sequence in order to fill its Roche
lobe. Remarkably, a distance of > 27 kpc is inferred by the companion's
luminosity and this is supported by the large observed systemic velocity (gamma
= 103 +- 4 km/s) which requires such a distance in order to be consistent with
the Galactic rotation curve.
