\begin{table}%t5
%\centering
\par
\caption{Number and percentage of radio quiet and radio loud \mbox{X-ray} 
detected QSOs in the V\'eron-Cetty \& V\'eron catalogue. Columns 1 and~2: $O_{\rm USNO}$ 
range, Col. 3: number of QSOs, Col. 4: number of sources brighter than 0.02 count 
s$^{-1}$ in the ROSAT catalogues, Col.~5: percentage of ROSAT detected QSOs; Col. 
6 to 8: the same for the radio loud QSOs.}
\label{xopt}  
\begin{tabular}{rr|rrr|rrr}
\hline \hline \multicolumn{2}{c}{} & \multicolumn{3}{c}{Radio quiet} & \multicolumn{3}{c}{Radio loud} \\ 
\hline
$O_{\rm min}$ & $O_{\rm max}$  & $n$ & $n_{\rm X}$ &  \% & $n$ & $n_{\rm X}$ & \%  \\
\hline
      &$<$15.5 &   80 &   58 & 72 &   9 &  7 & 78 \\
 15.5 &   16.0 &   90 &   48 & 53 &  21 & 17 & 81 \\
 16.0 &   16.5 &  243 &  101 & 42 &  49 & 37 & 76 \\
 16.5 &   17.0 &  433 &  144 & 33 &  99 & 60 & 61 \\
 17.0 &   17.5 &  679 &  101 & 15 & 172 & 77 & 45 \\
 17.5 &   18.0 &  900 &   60 &  7 & 201 & 59 & 29 \\
 18.0 &   18.5 &  977 &   23 &  2 & 169 & 41 & 24 \\
 18.5 &   19.0 & 1040 &   13 &  1 & 107 & 11 & 10 \\
 19.0 &   19.5 &  475 &    4 &  1 &  41 &  5 & 12 \\
 19.5 &   20.0 &   87 &    0 &  0 &  18 &  5 & 28 \\
\hline
\end{tabular}
\end{table}