1998A&A...332..661D

We discuss the determination of carbon isotopic ratios ¹²C/¹³C in N-type Galactic carbon stars. Ohnaka & Tsuji (1996A&A...310..933O) reported ¹²C/¹³C ratios smaller by a factor 2 or 3 than the determinations of Lambert et al. (1986ApJS...62..373L). Using synthetic spectra of carbon stars, we analyse systematic errors in their iso-intensity method. It is found to be rather sentitive to model parameters and blends. Furthermore, there are large discrepancies between the effective temperatures adopted by these two studies (mean discrepancy close to 260K for the 20 stars in common). That, together with uncertainties due to model atmospheres, to carbon enrichments and effects of the blends, could explain most of the discrepancies between these two studies. The LGEH86 analysis is, on the other hand, rather insentitive to model parameters. We therefore conclude that large ¹²C/¹³C ratios in cool Galactic carbon stars should be favoured.