\begin{table}%TA1
\caption{Associated \os\ systems.}\label{table_assoc}      
%\centerline
 {
\begin{tabular}{lcccc}
\hline\hline   
 Ion   & $z$      &$\log N$(A)$^a$ &$\log N$(B)$^a$   &QSO\\ 
\hline
 {\ho} & 2.300 & 16.36(0.23)$^b$&16.53(0.23)$^a$&\he\\ 
 {\cf} &       & 14.01(0.02) & 14.32(0.02) &\\
 {\os} &       & 14.69(0.07)& 14.73(0.03) &\\
\hline
 {\ho} & 2.314 & 15.80(0.01)&15.33(0.04)&\he\\ 
 {\cf} &       & 12.77(0.01) & 12.82(0.01) &\\
 {\os} &       & 13.32(0.02)& 13.38(0.04) &\\
\hline
 {\ho} & 2.776 & 15.85(0.05)&...&\rx\\ 
 {\cf} &       & 14.10(0.03) &14.20(0.10)  &\\
 {\os} &       & 14.77(0.02)&14.75(0.03)  &\\
\hline
\end{tabular}}
\smallskip
$^a$ Total column densities. 
$^b$ From Lopez et~al. (1999).
\end{table}