1996A&A...306..241G

We present CO and HCN observations of carbon stars. They consist of partly new detections in the ¹²CO J=(1-0),(2-1) and HCN(1-0) lines obtained with the SEST and the IRAM telescope, and of ¹²CO and ¹³CO J=(1-0),(2-1) and (3-2) observations with IRAM and the JCMT of some of the infrared brightest carbon stars. Recently, Bujarrabal et al. (1994a,b) proposed a critical value for the ratio of the integrated intensities HCN(1-0)/¹²CO(1-0) to distinguish between carbon stars and oxygen-rich stars. In four carbon stars we find ratios consistent with their borderline value, and in two cases HCN/CO ratios below their suggested borderline value in stars which are almost certainly carbon stars. The apparent discrepancy may lie in the fact that the critical line ratio in Bujarrabal et al. is derived for the 30m IRAM telescope, while our ratios are derived from 15m SEST data. The difference in beam size may result in slightly different excitation regions being sampled.