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 ( \sim 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 \sim 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 .