Top panel: relation between the seed photon temperature T_seed of the comt (Comptonised optical-UV/soft X-ray excess) component and the observed UV flux in the UVW2 filter. Bottom panel: relation between the normalisation of comt and the observed UVW2 flux. The comt normalisation is shown in units of 10⁵⁶ photons s^-1 keV^-1. The data in the above panels have been fitted with linear functions, shown in dashed black lines, as described in Sect. 3.1.