Spectroscopy of the dwarf nova TW Virginis caught on the rise to outburst

Time-resolved spectroscopic observations of the dwarf nova TW Vir over a five-day period are presented. During this time, the system evolved from a quiescent state to outburst. There was a weakening in the emission lines and a slow rise in the continuum prior to the rapid rise of the outburst. The actual transition between these two phases was observed for the first time in any dwarf nova. These data were compared to Mineshige's model in which the main outburst is preceded by a warm phase. A simple model was constructed which mimics the time-dependent behavior of the more elaborate Mineshige models. The agreement with the observations is quite good. 23 refs.