Stack of STIS (left column) and COS (right column) HST spectra of CO absorption lines towards LMC and SMC stars. The black lines with blue error bars represent the CO bands co-added spectrum (using R0 line as a reference line for each band to define the velocity offset), while the red lines correspond to the co-added fit profiles. Each spectrum is arbitrary shifted in y-axis for illustrative purposes. The name of each background star is provided above each spectrum. This includes three sightlines Sk −67 5, Sk −68 135, and Sk −69 246, where Bluhm & de Boer (2001), André et al. (2004) previously claimed CO detection, while HST data indicate non-detection with much lower upper limits on CO column densities, see Table C.1.