Montage of CO spectra of S stars in which ¹³CO lines are visible. The main beam (observed) temperature, T_mb, is the ordinate in units of mK. ¹³CO and ¹²CO transitions are identified in the upper left corners of each panel along with the integrated line fluxes and the rms of T_mb. More saturated lines tend to exhibit flatter profile tops. Note that the relatively unsaturated ¹³CO lines invariably show double peaks. This is characteristic of uniform spherical expansion in a partially resolved envelope (see text).