Optical and X-ray light curve of M31N 2007-12b assuming a time of nova outburst of 2007 December 9.0 UT. Optical white light (diamonds) and R filter (triangles) apparent magnitudes are plotted using data from the CBAT M 31 (Apparent) Novae Page and additional Super-LOTIS data (see Sect. 2.1, left y-axis). X-ray luminosities and upper limits derived for an absorbed black-body spectrum, are plotted as squares (see Table 1, right y-axis). Detections are connected with solid lines, and upper limits with dotted lines. Statistical errors in the XMM-Newton EPIC pn data points are smaller than the symbol size. For the four XMM-Newton bright state observations, we also give the bolometric luminosities derived for the WD atmosphere model with solar abundances (stars, see Table 5).