Best-fitting spectrum of the fiducial model, compared to the observed spectrum of DENIS J0255. Upper panels: similar to Fig. 3, the observed spectrum is plotted in black. The orange line displays the model spectrum and the second panel shows the residuals between the observed and model spectra in black. The mean photon noise of each order is indicated with a black error bar. The orange error bar shows the modelled uncertainty, defined as $\langle \sqrt{\mathrm{diag}\left(s^2{\Sigma }_0\right)}\rangle$ per order. Lower panels: zoom-in of the sixth spectral order, showing several correlated residual structures in the bottom panel.