Black: Coadded CO (left) and CH₄ (right) line, obtained from coaddition of the observed spectrum within  ± 0.5 nm of (i) the R2–R7 and P1–P3 lines of CO and (ii) the 12 CH₄ features identified by dots in Fig. 1. Colored lines: result of the same treatment applied to synthetic spectra. In the right panel, the feature at –0.13 nm is mostly due to the telluric counterpart of the coadded methane line.