2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/1029We 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. (1986). 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.Accepted for publication in A&AAstrophysicstext