  In some recent works, the C/O abundance ratio in high-metallicity stars with
planets is found to vary from 0.4 to about 1.0. This has led to discussions
about the existence of terrestrial planets with a carbon-dominated composition
that is very different from the composition of the Earth. The C/O values were
obtained by determining carbon abundances from high-excitation CI lines and
oxygen abundances from the forbidden [OI] line at 6300 A. This weak line is,
however, strongly affected by a nickel blend at high metallicities. Aiming for
more precise C/O ratios, oxygen abundances in this paper are derived from the
high-excitation OI triplet at 7774 A and carbon abundances from the CI lines at
5052 and 5380 A using MARCS model atmospheres and including non-LTE
corrections. The results do not confirm the high C/O ratios previously found.
C/O shows a tight, slightly increasing dependence on metallicity from C/O=0.58
at [Fe/H]=0.0 to C/O=0.70 at [Fe/H] =0.4 with an rms scatter of only 0.06.
Assuming that the composition of a proto-planetary disk is the same as that of
the host star, the C/O values found in this paper lend no support to the
existence of carbon-rich planets. The small scatter of C/O among thin-disk
stars suggests that the nucleosynthesis products of Type II supernovae and low-
to intermediate-mass stars are well mixed in the interstellar medium.
