Positions of the source in the T_eff − L plane according to different XMM-Newton (upper panel) and Chandra (lower panel) observations. The WD mass was fixed, and different colours indicate different masses from 0.95 to 1.35 M_⊙. The fits with M = 1.1 M_⊙ are additionally indicated by the increased marker size. Model dependencies were taken from Wolf et al. (2013). Only the model curves with steady-state thermonuclear burning are shown. The numbers denote the spectrum number (see Sect. 2).