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 \pm 5 km s ^ { -1 } and its rotational velocity to be v \sin i = 138 \pm 5 km s ^ { -1 } .
We also measure the radial velocity semi-amplitude of the primary star to be K _ { R } = 170 \pm 10 km s ^ { -1 } .
From these parameters we obtain a mass of 1.04 \pm 0.06 M _ { \odot } for the white dwarf primary star and a mass of 0.87 \pm 0.06 M _ { \odot } for the G8 V secondary star .
The inclination of the system is found to be 82.2 \pm 3.2 ^ { \circ } and we estimate that the system lies at a distance of 1700 \pm 300 pc .
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 .