X-Shooter spectrum of WISE J0725−2351 (black; telluric absorption bands are marked by ⊕) compared to a synthetic spectrum with [Fe/H] = −2.0, T_eff = 6250 K and log g = 4.0 (red). Top: for the full wavelength interval additional synthetic spectra with changed temperature T_eff = 6500 K (green) and 6000 K (blue) are shown. Bottom: for the region of the Balmer jump, the additional synthetic spectra show the effect of changing gravity to log  g = 4.5 (green) and log  g = 3.5 (blue).