for large carbon abundances, in that the additional C opacity can alter the atmospheric structure significantly, and therefore the derived stellar parameters. The impact of this additional C opacity has so far been explored only in the case of the field He-sdB star PG1544$+$488. For this star Lanz et al. ([@labr03]) found that the value of [$\log {\rm g}$]{} given by NLTE models with a C abundance of 0.02 by mass, as obtained by fitting the UV C III lines, was about 0.5 dex larger than the value given by models without C. Additional calculations are needed to determine the carbon abundance at which it is possible to neglect carbon in the model atmosphere and still derive accurate stellar parameters. We note, however, that we find a supersolar carbon abundance in only one star (W20334), and so our crude approach is likely adequate for the remaining stars.

The comparison between model spectra and observations resulted in a carbon abundance compatible with the cluster abundance (\[Fe/H\] $=
-1.15$, Harris [@harr96]) except for the three most helium-rich stars W14040, W18899, and W20334. For W14040 ([${\rm \log{\frac{n_{He}}{n_{H}}}}$]{}= $-0.31$) and W18899 ([${\rm \log{\frac{n_{He}}{n_{H}}}}$]{}= $-0.21$) we estimate logarithmic carbon mass fraction relative to the Sun 