Top panel: the best-fit X-ray power-law luminosity calculated between 0.2–10 keV, plotted versus the disc blackbody bolometric luminosity for the 10 XMM-Newtonobservations. Bottom panel: 0.2–10 keV luminosity of the soft excess modelled by warm Comptonisation, plotted versus the disc blackbody bolometric luminosity for the 10 XMM-Newtonobservations. The luminosities are obtained from the broad-band modelling described in Sect. 5. The blue line in the bottom panel is a fit to the luminosities with linear model: L_{soft excess} = 0.448 L_disc − 1.007, where the luminosities are in units of 10⁴⁴ erg s^-1.