\begin{table} %t4
\par
\caption{CNO abundances used in model photospheres.\label{tab4}}
%\centerline
 {\begin{tabular}{ l c c l}
\hline \hline
   & Case a & Case b  \\    
\hline
\noalign{\smallskip}
 log $A_{\rm C}^{a} $    &  8.35      &   8.08      \\
 log $A_{\rm N}^{b}$    &   8.11     &    8.45     \\
 log $A_{\rm O}^{c}$    &   8.69      &   8.69      \\
\hline 
\end{tabular}
}
\par
\smallskip
$^{a}$   Tsuji (\cite{Tsuji91}); 
$^{b}$   Aoki \& Tsuji (\cite{Aoki97});
$^{c}$   Allende Prieto et~al. (2002).  
\end{table}