SIMBAD references

We present observations of circumstellar millimetre-wave ¹³CO line emission towards a sample of 20 cool carbon stars. Using a detailed radiative transfer model we estimate the circumstellar ¹²CO/¹³CO-ratios, which we believe accurately measure the important stellar ¹²C/¹³C-ratios. For those optically bright carbon stars where it is possible, our derived ¹²C/¹³C-ratios are compared with the photospheric results, obtained with different methods. Our estimates agree well with those of Lambert et al. (1986ApJS...62..373L) It is shown that a straightforward determination of the ¹²CO/¹³CO-ratio from observed line intensity ratios is often hampered by optical depth effects, and that a detailed radiative transfer analysis is needed in order to determine reliable isotope ratios.