  In order to investigate the carbon-to-oxygen ratio of the young open cluster
M45 (Pleiades), the C and O abundances of selected 32 F-G type dwarfs (in the
effective temperature range of Teff~5800-7600K and projected rotational
velocity range of vesini~10-110km/s) belonging to this cluster were determined
by applying the synthetic spectrum-fitting technique to C I 5380 and O I 6156-8
lines. The non-LTE corrections for these C I and O I lines were found to be
practically negligible (less than a few hundredths dex). The resulting C and O
abundances (along with the Fe abundance) turned out nearly uniform without any
systematic dependence upon Teff or vesini. We found, however, in spite of
almost solar Fe abundance ([Fe/H]~0), carbon turned out to be slightly subsolar
([C/H]~-0.1) while oxygen slightly supersolar ([O/H]~+0.1). This lead to a
conclusion that [C/O] ratio was moderately subsolar (~-0.2) in the primordial
gas from which these Pleiades stars were formed ~120--130 Myr ago.
Interestingly, similarly young B-type stars are reported to show just the same
result ([C/O]~-0.2), while rather aged (~1-10 Gyr) field F-G stars of
near-solar metallicity yield almost the solar value ([C/O]~0) on the average.
Such a difference in the C/O ratio between two star groups of distinctly
different ages may be explained as a consequence of orbit migration mechanism
which Galactic stars may undergo over a long time.
