\begin{table}%t4
\par
\caption {\label{tab_abundances}Mean NLTE abundances (on the scale in which $\log \epsilon$[H]~=~12) and total 1-$\sigma$ uncertainties (in brackets), in units of the last digits.}
%\centerline
 {\begin{tabular}{lcccc} \hline\hline
                         & HD~180~642   & B stars      & Sun 1-D    & Sun 3-D\\\hline
He/H(4)                  & 0.088(18)    & $\sim$0.089  & 0.085(1)   & 0.085(2)\\
$\log \epsilon$(C)\ (9)  & 8.21(10)     & $\sim$8.20   & 8.52(6)    & 8.39(5)\\ 
$\log \epsilon$(N) (21)  & 8.00(19)     & $\sim$7.79   & 7.92(6)    & 7.78(6)\\ 
$\log \epsilon$(O) (25)  & 8.53(14)     & $\sim$8.58   & 8.83(6)    & 8.66(5)\\ 
$\log \epsilon$(Ne) (6)  & 7.86(16)$^a$ & $\sim$8.07   & 8.08(6)    & 7.84(6)\\ 
$\log \epsilon$(Mg) (1)  & 7.34(20)     & $\sim$7.48   & 7.58(5)    & 7.53(9)\\ 
$\log \epsilon$(Al) (3)  & 6.22(15)     & $\sim$6.08   & 6.47(7)    & 6.37(6)\\ 
$\log \epsilon$(Si) (7)  & 7.19(19)     & $\sim$7.20   & 7.55(5)    & 7.51(4)\\ 
$\log \epsilon$(S) (4)   & 7.10(34)     & $\sim$7.19   & 7.33(11)   & 7.14(5)\\ 
$\log \epsilon$(Fe) (21) & 7.34(21)     & $\sim$7.36   & 7.50(5)    & 7.45(5)\\
\hline
${\rm [N/C]}$            &  --0.21(22)  & $\sim$--0.41 & --0.60(9)  & --0.61(8)\\
${\rm [N/O]}$            &  --0.53(24)  & $\sim$--0.79 & --0.91(9)  & --0.88(8)\\
\hline
$Z$                      & 0.0099(16)   & $\sim$0.0108 & 0.0172(12) & 0.0124(7)\\
\hline
\end{tabular}}
\par
\smallskip
 {The number of spectral lines used is given in brackets in the first column. For comparison purposes, we provide typical values found for early B~dwarfs in the solar neighbourhood (Morel~\cite{morel09}, and references therein; B~stars), the standard solar composition of Grevesse \& Sauval\ (\cite{grevesse_sauval}, Sun 1-D), and updated values derived from three-dimensional hydrodynamical models (Asplund 
 et~al.\ \cite{asplund}, Sun 3-D). We define [N/C] and [N/O] as $\log$[$\epsilon$(N)/$\epsilon$(C)] and $\log$[$\epsilon$(N)/$\epsilon$(O)], respectively. The metallicity, $Z$, is given in the last row. To compute this quantity, we assumed the abundances of Grevesse \& Sauval\ (\cite{grevesse_sauval}) for the trace elements not under study.\\
$^a$ A negligible difference amounting to 0.01~dex is obtained when using the $T_{\rm eff}$ derived from the Ne ionisation balance (Morel \& Butler\ \cite{morel_butler}).} 
\end{table}