BT Monocerotis (Nova Monocerotis 1939) was a nova, which lit up in the constellation Monoceros in 1939. It was discovered on a spectral plate by Fred L. Whipple on December 23, 1939. BT Monocerotis reached a brightness of 4.5 mag. Its brightness decreased after the outbreak in 36 days by 3 mag. The light curve for the eruption had a long plateau period.
Photographic plates taken for 30 years prior to the eruption show that BT Monocerotis remained visible during that period. Prior to 1933, BT Monocerotis had an average magnitude of 15.52 with a variation of 1.2 magnitudes. It retained the same magnitude up until the eruption, showing a variation of 0.9 magnitudes. Thus it did not show a pre-eruption rise in brightness.
This is an interacting binary star system consisting of a white dwarf primary star and a main sequence star with a stellar classification of G8V. The orbit has a period of 0.33381379 days and an inclination of 88.2° to the line of sight to the Earth, resulting in an eclipsing binary. The nova eruption is believed to have been driven by mass transferred from the secondary star to the white dwarf. It remains uncertain whether the white dwarf has an accretion disk formed by this material. Matter outflowing from the system has a line of sight velocity of 450 km s−1, but may be moving at up to 3,200 km s−1 if the flow is strictly bipolar.