VLT/X–shooter average spectrum (black, upper panel UVB, central panel VIS, bottom panel NIR) of SDSS1238. Regions strongly affected by telluric absorption are drawn in grey. The two insets show the white dwarf Mg ii (4481 Å) and the secondary K i (⁠|$12\, 432/12\, 522$| Å) absorption features used to measure their radial velocities. For comparison, the SDSS spectrum is shown in blue (no offset in flux has been applied). The agreement between the two spectra illustrates the good relative and absolute flux calibration of the X-shooter data. The circles and the squares are the SDSS (blue, green, red, purple, and salmon for the u, g, i, r, and |$z$| band filters) and 2MASS (grey, gold and cyan for the J, H, K_s band filters) photometry (obtained at different epochs).