\begin{table}%t4
\caption{Mean NLTE abundances (on the scale in which $\log \epsilon$[H]~= 12) and total 1$\sigma$-uncertainties (values for \xcma{} from Morel et~al. \cite{morel}). The number of spectral lines used is given in brackets. A blank indicates that the abundance of a given element could not be determined. For comparison purposes, we provide in the last column the typical values found for OB~dwarfs in the solar neighbourhood (Daflon \& Cunha \cite{daflon_cunha}; Gummersbach et~al. \cite{gummersbach};  \mbox{Kilian-Montenbruck} et~al. \cite{kilian_montenbruck}). We define [N/C] and [N/O] as $\log$~[$\epsilon$(N)/$\epsilon$(C)] and $\log$~[$\epsilon$(N)/$\epsilon$(O)], respectively.}
\label{tab:abundances}
\small%\centerline
 {
\begin{tabular}{lcccc}
\hline
\hline
&&&& \\[-8pt]
                    & \multicolumn{1}{c}{\xcma{}} & \multicolumn{1}{c}{\tsco{}} & \multicolumn{1}{c}{\tcar{}} & OB stars\\
\hline
He/H                & 0.098   $\pm$  0.017 (10) & 0.085   $\pm$   0.027 (9) & 0.083   $\pm$   0.028 (5)    &  0.10$^a$ \\  
$\log \epsilon$(C)  & 8.18    $\pm$   0.12 (9)   & 8.19    $\pm$   0.14 (15) & 7.16    $\pm$   0.46 (1)$^b$ &  $\sim$8.2 \\    
$\log \epsilon$(N)  & 8.00    $\pm$   0.17 (34)  & 8.15    $\pm$   0.20 (35) & 8.56    $\pm$   0.27 (12)    &  $\sim$7.6 \\    
$\log \epsilon$(O)  & 8.59    $\pm$   0.17 (34)  & 8.62    $\pm$   0.20 (42) & 8.38    $\pm$   0.22 (7)     &  $\sim$8.5 \\    
$\log \epsilon$(Mg) & 7.37    $\pm$   0.20 (1)   & 7.45    $\pm$   0.09 (2)  &                           &  $\sim$7.4 \\    
$\log \epsilon$(Al) & 6.16    $\pm$   0.22 (4)   & 6.31    $\pm$   0.29 (3)  &                            &  $\sim$6.1 \\    
$\log \epsilon$(Si) & 7.13    $\pm$  0.21 (4)   & 7.24    $\pm$   0.14 (9)  & 7.43    $\pm$   0.23 (10)    &  $\sim$7.2 \\    
$\log \epsilon$(S)  & 6.99    $\pm$   0.16 (2)   & 7.18    $\pm$   0.28 (3)  & 7.32    $\pm$   0.33 (1)     &  $\sim$7.2 \\    
$\log \epsilon$(Fe) & 7.30    $\pm$   0.22 (32)  & 7.33    $\pm$   0.31 (13) &                            &  $\sim$7.3$^c$ \\
\hline		      			 ${\rm [N/C]}$       & --0.18    $\pm$   0.21     & --0.04   $\pm$   0.25     & +1.40   $\pm$   0.53         &  $\sim$--0.6 \\  
${\rm [N/O]}$       & --0.59   $\pm$   0.25     & --0.47   $\pm$ 0.29     & +0.18   $\pm$   0.35         &  $\sim$--0.9 \\  
\hline
\end{tabular}}
\medskip
$^a$ From Lyubimkov et~al. (\cite{lyubimkov});
$^b$~derived from a differential analysis of the \ion{C}{ii}~$\lambda$4267 doublet with respect to \tsco{} (see text);
$^c$~mean NLTE abundance from Morel et~al. (\cite{morel}).
\end{table}