\begin{table}%t12
  \caption{Comparison of the abundance ratios of carbon, nitrogen and
  oxygen.}
  \label{rat_comp}
%\begin{center}
\par
\begin{tabular}{c c c c c}
      \hline
    \hline
    & \bd & \nsix & Sun & O Stars\\
    \hline
    N/O & 0.24 & 0.42 & 0.19 & 0.15 \\
    C/O & 1.59 & 0.45 & 0.72 & 0.42\\
    \makebox{\rule[-0.2cm]{0cm}{0.55cm}$\frac{{\rm (C+N+O)}}{{\rm H}}$}(--4)  & 13.0 & 10.3 & 9.4 & 6.56\\
    \hline
  \end{tabular}
%\end{center}
\par
\end{table}