CO AX bands at z_abs = 2.1422 towards J2331−0908 (top four panels, AX(3–0) is not covered by the instrument setup). The bottom-right panel shows a co-addition of the CO bands, using -weighting, where f is the oscillator strength and σ the uncertainty on the normalised flux, for easy visualisation of the detection. The bottom-left panel shows the curves (greyfor individual band, black for total).