Observed spectrum of CFBDSIR 2149 in black in each spectral band. Best fitting BT-Settl models in absolute flux (W m² μm^-1) are represented in colour. The spectra presented here have been binned to R ~ 200. 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).