\begin{table}%t3
\caption{\label{neb_ab}Element abundances used in the NLTE model atmospheres.}
\par
%\centering
\small
\begin{tabular}{cc}
\hline\hline
\multicolumn{2}{c}{Mass fraction} \\
 \hline \noalign{\smallskip}
H & $6.808\times10^{-1}$\\
He & $3.176\times10^{-1}$\\
C & $9.104\times10^{-5}$\\
N & $1.061\times10^{-3}$\\
O & $4.205\times10^{-4}$\\
\hline
\end{tabular}
\end{table}