Triple-Gaussian fit of the CO (16–15) transition (red line, from Fig. 2): blueshifted (light blue line), ambient (pink line), redshifted (grey line) components, and total fit (black line). An additional component has been fitted with respect to the OH line in Fig. A.1: a narrow CO feature at 10.7 km s^-1 due to mesospheric CO over-compensated by the correction for the atmospheric opacity and well reproduced by a fourth Gaussian (yellow line). The vertical dashed line is at the cloud ν_lsr of − 11.2 km s^-1 (Narayanan & Walker 1996; Gómez et al. 1999).