Q. How good are the signal-free reference positions used when position or beam switching?

The reduced SWAS spectra give the difference between an on-source position and a signal-free reference. The reference positions were checked for ¹²CO line emission using FCRAO observations and literature data. Although we carefully checked the reference positions for emission, we cannot exclude the possibility that some of these reference positions suffer from occasional low-level contamination. However, this contamination is found for only for a few sources and is obvious only in the co-added data of deeply integrated [CI] observations. As the [CI] spectra are observed simultaneously with the H₂O, O₂, and ¹³CO 5-4 transitions, the long integration times used in the search for 0₂ and H₂O emission have resulted in deeply integrated [CI] spectra.