Observed spectrum of CFBDSIR 2149 in black in each spectral band and around the K i doublet. Best fitting Ma&Mo2017 models after normalisation on the J band peak are represented in colour. Noise from the observed spectrum is represented by the black dotted line. From top down and left to right the panels show the spectra in z at R ~ 300, and at R ~ 600 at all other wavelengths. The spectra presented here are normalised to their local peak intensity. Blue: best fitting Ma&Mo2017 low-gravity model (775 K, log g = 4.0, [M/H] = 0) Red: best fitting Ma&Mo2017 model (775 K, log g = 4.5, [M/H] = 0). Green: best fitting Ma&Mo2017 high-metallicity model (800 K, log g = 5.0, [M/H] = +0.5).