Calibrated spectrum of DENIS J0255, observed with CRIRES⁺ in the K2166 wavelength setting. The y axes indicate the flux F_λ in units of erg s⁻¹ cm⁻² nm⁻¹ and the x axes denote the wavelength λ in nm. Top panel: spectrum over the full wavelength range, with the black line indicating the spectrum after the calibration described in Sect. 3.1. The grey line shows the observed spectrum without correcting for telluric absorption and removing outliers. Bottom panel: zoom-in of the sixth spectral order, presenting numerous rotationally broadened spectral lines of ¹²CO.