1. The dwarf nova BZ UMa Simultaneous photometry and echelle- spectroscopy during the 2005 January Outburst MNRAS, 2006, Coming Soon Vitaly Neustroev (NUI Galway ), S. Zharikov and R. Michel

2. BZ UMa Period: 97.8 min Period: 97.8 min Outburst cycle: > 300 d Outburst cycle: > 300 d Min: V≈16-17 (17.8) Min: V≈16-17 (17.8) Max: V≈10.5-11.5 Max: V≈10.5-11.5 Very strong Balmer emission lines Very strong Balmer emission lines

3. Spectrum

4. Doppler Tomography April, 2004. Quiescence: H α April, 2004. Quiescence: H α

5. Observations OAN SPM (Mexico) OAN SPM (Mexico) Photometry: 2005 Jan 12-13 and 15-17 Photometry: 2005 Jan 12-13 and 15-17 Echelle-spectroscopy: 2005 Jan 15-17 Echelle-spectroscopy: 2005 Jan 15-17

6. Light Curve

10. Colours

11. Line profiles

12. FWHM, Fluxes and EW of Hα

13. Doppler Tomography April, 2004. Quiescence: H α April, 2004. Quiescence: H α

14. Doppler Tomography Jan 15, 2005. Preoutburst: H α Jan 15, 2005. Preoutburst: H α

15. Doppler Tomography Jan 16, 2005. Outburst, 1 st stage: H α Jan 16, 2005. Outburst, 1 st stage: H α

16. Doppler Tomography Jan 16, 2005. Outburst, 3 rd stage: H α Jan 16, 2005. Outburst, 3 rd stage: H α

17. Doppler Tomography Jan 17, 2005. Outburst, Decline: H α Jan 17, 2005. Outburst, Decline: H α

18. Doppler Tomography Jan 17, 2005. Outburst, Decline: H β Jan 17, 2005. Outburst, Decline: H β

19. Doppler Tomography Jan 17, 2005. Outburst, Decline: He I 5876 Jan 17, 2005. Outburst, Decline: He I 5876

20. Inside-Out or Outside-In outburst? The only way to be sure of the outburst type is from eclipse profile observations during outbursts and BZ UMa is not an eclipsed system. The only way to be sure of the outburst type is from eclipse profile observations during outbursts and BZ UMa is not an eclipsed system. A very low mass-transfer rate for BZ UMa (≤10 15 g/s) implies that theoretically, its outbursts should be of an inside-out type. A very low mass-transfer rate for BZ UMa (≤10 15 g/s) implies that theoretically, its outbursts should be of an inside-out type.

21. Line profiles

24. Summary We reported 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. We reported 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. Using Doppler tomography we found that the unusual emission distribution detected in quiescence held during the outburst. Using Doppler tomography we found that the unusual emission distribution detected in quiescence held during the outburst. On the base of the spectral changes during the rising stage we conclude that the 2005 January outburst of BZ UMa was of the inside-out type. On the base of the spectral changes during the rising stage we conclude that the 2005 January outburst of BZ UMa was of the inside-out type. Nevertheless, a highly asymmetrical light curve with a “jump” shows that this outburst cannot easily be fit into the framework of the current Disc Instability Model. Nevertheless, a highly asymmetrical light curve with a “jump” shows that this outburst cannot easily be fit into the framework of the current Disc Instability Model.