Carbon planets rarer than previously thought?

On the Carbon-to-Oxygen Ratio Measurement in Nearby Sunlike Stars: Implications for Planet Formation and the Determination of Stellar Abundances

There do not appear to be enough cool carbon-rich stars that would be expected if the proportion of stars with high C/O ratios is as great as has been previously estimated. This may indicate problems with the way these ratios are determined. If so, carbon planets may be rarer than previously thought.

The carbon-to-oxygen ratio in stars with planets
http://arxiv.org/abs/1303.1726

from the abstract:

Quote :

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.

55 Cancri is mentioned in the introduction but is not one of the stars listed in the tables at the end.