Observed spectrum of CFBDSIR 2149 in black in each spectral band and around the K i doublet. 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 on their local peak intensity. Best fitting BT-Settl models models in absolute flux are represented in colour. Blue: model closest to the best fitting solution in absolute flux after interpolation on both chi² maps (700 K, log g = 3.5, [M/H] = 0). Red: model closest to the best fitting solution in absolute flux after interpolation on the high-metallicity chi² map (800 K, log g = 5.0, [M/H] = 0.3). Green: best fitting model in absolute flux without interpolation (650 K, log g = 3.5, [M/H] = 0.3).