CO(2–1) line profiles of 5 red supergiants in RSGC1. The ALMA data are plotted as red histograms. Synthetic line profiles (see Sect. 3) are overplotted as black solid lines. The grey dashed regions indicate frequency regions that are contaminated by the ISM, a strong noise background, or other genuine emission (see Appendix A). The vertical dashed blue lines indicate the local standard of rest velocity, v_LSR, as deduced from the ALMA CO data (see Appendix A). The dotted and dashed-dotted blue lines indicate the v_LSR value as determined by Davies et al. (2008) and Nakashima & Deguchi (2006), respectively (see Table A.1). The noise in the spectrum is 1.2–1.4 mJy (see text for derivation) and is shown as (red) error bar in each panel. The dotted black line in the upper left panel is an alternative fit to the CO(2–1) line profile of F01, as discussed in Appendix C.