THE MASS OF THE WHITE-DWARF IN THE OLD NOVA BT MON

We present spectrophotometry of the eclipsing old nova BT Mon (Nova Mo n 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(-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 +/- 10 km s(-1). From these parameters we obtain a ma ss of 1.04 +/- 0.06 M. for the white dwarf primary star and a mass of 0.87 +/- 0.06 M. for the G8 V secondary star. The inclination of the s ystem is found to be 82.degrees 2 +/- 3.degrees 2 and we estimate that the system lies at a distance of 1700 +/- 300 pc. The high mass of th e white dwarf and our finding that BT Mon was probably a fast nova tog ether constitute a new piece of evidence in favour of the thermonuclea r runaway model of classical nova outbursts. The emission lines are si ngle-peaked throughout the orbital cycle, showing absorption around ph ase 0.5, high-velocity S-wave components and large phase offsets in th eir radial velocity curves. In each of these respects, BT Mon is simil ar to the SW Sex stars. We also find quasi-periodic flaring in the tra iled spectra, which makes BT Mon a candidate intermediate polar.