The mass of the white dwarf in the old nova BT

We present spectrophotometry of the eclipsing old nova BT Mon (Nova Mon 1939). By detecting weak absorption features from the secondary star, we nd its radial velocity semi-amplitude to be K R = 205 5 km s ?1 and its rotational velocity to be v sin i = 138 5 km s ?1. We also measure the radial velocity semi-amplitude of the primary star to be K R = 170 10km s ?1. From these parameters we obtain a mass of 1:040.06M for the white dwarf primary star and a mass of 0:870.06M for the G8 V secondary star. The inclination of the system is found to be 82:2 3:2 and we estimate that the system lies at a distance of 1700300pc. The high mass of the white dwarf and our nding 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 oosets in their radial velocity curves. In each of these respects, BT Mon is similar to the SW Sex stars. We also nd quasi-periodic aring in the trailed spectra, which makes BT Mon a candidate intermediate polar.