Positions of the source in the T_eff − log g plane according to different XMM-Newton (upper panel) and Chandra (lower panel) observations (see Tables 3 and 4). The WD mass was fixed, and different colours indicate different masses from 0.95 to 1.35 M_⊙. For clarity, only a portion of the mass range is shown. The fits with M = 1.1 M_⊙ are additionally indicated by the increased marker size. Model dependencies for various WD masses, taken from Nomoto et al. (2007), are shown by black dashed curves. The numbers at the curves indicate WD masses (in solar masses). The lower boundary of the stable thermonuclear burning band is shown by the dashed red line. The Eddington limit for solar H/He abundances is shown by the blue dashed line. The numbers denote the spectrum number (see Sect. 2).