  Based on the medium resolution near infrared spectra of 13CO (3,1) band,
carbon isotopic ratios are estimated in 48 M dwarfs, for which we had
determined the carbon and oxygen abundances from CO and H2O lines,
respectively. We find clear evidence for the presence of a 13CO feature for the
first time in the spectra of M dwarfs. The spectral resolution of our observed
data, however, is not high enough to analyze the 13CO feature directly.
Instead, we compare observed spectrum with synthetic spectra assuming 12C/13C =
10, 25, 50, 100, and 200 for each of 48 M dwarfs and estimate the best possible
12C/13C ratio by the chi-square analysis. The resulting 12C/13C ratios in M
dwarfs distribute from 39 to a lower limit of 200. The mean value of 31 M
dwarfs for which 12C/13C ratios are determined is 12C/13C = 87 +- 21 (p.e.),
and that of 48 M dwarfs including those with the lower limit of 200 is 12C/13C
> 127 +- 41 (p.e.). These results are somewhat larger than the 12C/13C ratio of
the present interstellar matter (ISM) determined from the molecular lines
observed in the millimeter and optical wavelength regions. Since the amount of
13C in the ISM has increased with time due to mass-loss from evolved stars, the
12C/13C ratios in M dwarfs, reflecting those of the past ISM, should be larger
than those of the present ISM. In M dwarfs, log 13C/12C plotted against log Ac
shows a large scatter without clear dependence on the metallicity. This result
shows a marked contrast to log 16O/12C (= log Ao/Ac) plotted against log Ac,
which shows a rather tight correlation with the larger value at the lower
metallicity. Such a contrast can be a natural consequence that 16O and 12C are
the primary products in the stellar nuclear synthesis while 13C is the
secondary product, at least partly.
