  We present spectrophotometry of the eclipsing old nova BT Mon (Nova Mon
1939). By detecting weak absorption features from the secondary star, we find
its radial velocity semi-amplitude to be K_R = 205+/-5 km/s and its rotational
velocity to be vsin i = 138+/-5 km/s. We also measure the radial velocity
semi-amplitude of the primary star to be K_R = 170+/-10 km/s. From these
parameters we obtain a mass of 1.04+/-0.06 M_sun for the white dwarf primary
star and a mass of 0.87+/-0.06 M_sun for the G8V secondary star. The
inclination of the system is found to be 82.2+/-3.2 deg and we estimate that
the system lies at a distance of 1700+/-300pc. The high mass of the white dwarf
and our finding that BT Mon was probably a fast nova together constitute a new
piece of evidence in favour of the thermonuclear runaway model of classical
nova outbursts. The emission lines are single peaked throughout the orbital
cycle, showing absorption around phase 0.5, high velocity S-wave components and
large phase offsets in their radial velocity curves. In each of these respects,
BT Mon is similar to the SW Sex stars. We also find quasi-periodic flaring in
the trailed spectra, which makes BT Mon a candidate intermediate polar.
