SIMBAD references

We have used the IRAM 30 m telescope on Pico de Veleta to observe 11 S-type stars in the J=1-0 and J=2-1 transitions of both ¹²C¹⁶O and ¹³C¹⁶O in order to derive the ¹²CO/¹³CO ratio in their circumstellar envelopes. Detections of high signal to noise were achieved for 7 stars. A mean isotope ratio of 35.3±6.2 was found. This ratio fits between the ratio for late M stars of about 12±3 and that of carbon stars of 58±17. Hence, we confirm the often proposed evolutionary sequence from late M through type S to carbon stars. Mass loss rates of 5x10^–8 to 3x10^–6M_☉/y are derived from the present CO data using a simple uniform outflow model with line-dependent opacities, in very good agreement with previous mass-loss estimates.