Recovered exoplanetary transmission spectrum of HD 209458 b from CRIRES observations of a single transit. The expected positions of CO lines are shown as dashed grey lines. Only a few of the 16 CO lines in the observed wavelength range are visible in the recovered spectrum. Inserted panel: stacked exoplanet spectra (normalized to average value in wings) over the expected position of CO lines (shown in red) and small sample (200) of 100 000 realizations of randomized positions of line centres (shown in turquoise). Only 1% of the randomized line positions show stronger line depth than the CO lines.