BT MONOCEROTIS - ANOTHER EXTRAORDINARY OLD NOVA

We present time-resolved spectrophotometry of the old nova BT Monocero tis (Nova Monocerotis 1939). While BT Mon's light curve possibly sugge sts the presence of an accretion disk, trailed spectrograms and Dopple r tomograms of the data provide no such evidence. The spectra do revea l, however, high-velocity gas motions near eclipse that are of a chara cter unique among other cataclysmic variables studied similarly to dat e. On the basis of the available data, we cannot describe the precise nature of BT Mon, but we suggest and discuss four possible interpretat ions of this extraordinary nova. Accretion by the system's white dwarf via the standard stream-disk mechanism is roughly supported by the ap pearance of BT Mon's light curve, yet our spectroscopy does not indica te the presence of an accretion disk. And while accretion by the white dwarf of a wind from the secondary star might explain the high-veloci ty gas motions we observe surrounding eclipse, our spectroscopy provid es no evidence for a wind. A third possible interpretation of our and other's data for BT Mon involves two mass-transfer streams that emanat e from the secondary star. One stream is the classical mass transfer s tream; the other is a supersonic stream driven from the secondary star by a de Laval nozzle. This two-stream model explains the appearance o f BT Mon's unusual, high-velocity gas motions visible during eclipse, but we find no additional evidence to support it. Finally, we consider the possibility that the system's white dwarf is highly magnetized, a ccretion occurring via an accretion column onto the primary star's sur face. Our observations mildly support such an interpretation, and rece nt results from other magnetic cataclysmic variables strengthen this l ast interpretation.