  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. (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.
