Corner plot of the parameters’ posterior distribution for the eRASS1 spectrum (left panel) and for the XMM-Newton RGS spectrum (right panel) of AT 2018bej. The WD mass, M, and radius, R, are considered as free parameters. The 2D contours correspond to 39.3, 86.5, and 98.9% confidence levels. The histograms show the normalised 1D distributions for a given parameter derived from the posterior samples. The hydrogen column density is fixed at N_H = 5.88 × 10²⁰ cm⁻² for the RGS spectrum. The best-fit parameter values are presented in Table 3.