Simultaneous photometry and echelle - spectroscopy of the dwarf nova BZ Ursae Majoris during the 2005 January

We report simultaneous photometric and echelle-spectroscopic observations of the dwarf nova BZ UMa during which we were lucky to catch the system at the onset of an outburst, the development of which we traced in detail from quiescence to early decline. The outburst had a precursor, and was of a short duration (∼5 days) with a highly asymmetrical light curve. On the rise we observed a 'jump' during which the brightness almost doubled over the course of half an hour. Power spectra analysis revealed well-defined oscillations with period of ∼42 minutes. Using Doppler tomography we found that the unusual emission distribution detected in quiescence held during the outburst. After the maximum a new emission source arose, from the inner hemisphere of the secondary star, which became the brightest at that time. We analyse this outburst in terms of 'inside-out' and 'outside-in' types, in order to determine which of these types occured in BZ UMa.