Normalised XP spectra. Top panel: mean normalised spectra of He-poor (red) and He-rich (blue) hot sds. Bottom panel: mean normalised spectra of cool (red) and hot (blue) hot sds. The locations of several relevant absorption lines are indicated in the bottom panel. Their wavelengths are 410, 434, and 486 nm (HI); 420, 454, 469 nm (HeII), and 588 nm (HeI). In both panels, the mean spectra are plotted together with 300 randomly selected normalised spectra from our full sample (black).