\begin{table}%T15
%%%%\centering


\par
\caption{Comparison of mean and median X-ray luminosities for NGC~2451~A and
B and other clusters of similar age, i.e., $\alpha$ Per and Pleiades (Prosser 
et~al. \cite{prosser96}) and NGC~6475 (Prosser et~al. \cite{prosser}).
The log $L_{\rm x}$ values are given separately for F-type stars
($0.31\le(B-V)_0<0.59$), G-type stars ($0.59\le(B-V)_0<0.82$),
K-type stars ($0.82\le(B-V)_0<1.41$), and M-type stars 
($(B-V)_0>1.41$). The values in parentheses indicate the number of stars 
used. Note that in the case of NGC~6475 the colour 
intervals are slightly different and that the K-type stars are divided
into early types (upper values) and late types (lower values).\label{clcomp}}
\begin{tabular}{lclclclcl}
\hline\hline
 & \multicolumn{2}{c}{F stars} & \multicolumn{2}{c}{G stars} &
   \multicolumn{2}{c}{K stars} & \multicolumn{2}{c}{M stars} \\
Cluster & mean & median & mean & median & mean & median & mean & median \\
\hline
NGC~2451 A  & 29.62$~\pm~$0.28 & 29.23 (8) & 29.91$~\pm~$0.14 & 29.83 (6) &
              29.49$~\pm~$0.18 & 29.02 (10)& 29.29$~\pm~$0.08 & 29.29 (8) \\
(incl.~upp.lim.)&            & 29.23 (8) &                & 29.46 (9) &
                             &$<$28.99 (15)&                & 29.29 (8) \\
NGC 2451 B  & 29.97$~\pm~$0.11 & 29.81 (14) & 29.95$~\pm~$0.08 & 29.82 (16) &
              29.77$~\pm~$0.06 & 29.71 (11) & --             & --    \\
\hline
$\alpha$ Per& 29.67$~\pm~$0.11 & 29.59& 29.70$~\pm~$0.10 & 29.68 &
              29.59$~\pm~$0.07 & 29.62& 29.05$~\pm~$0.04 & 28.98 \\
Pleiades    & 29.34$~\pm~$0.08 & 29.33& 29.37$~\pm~$0.11 & 29.21 &
              29.33$~\pm~$0.06 & 29.38& 28.98$~\pm~$0.04 & 28.99 \\
            &                &      &                &       &
              29.57$~\pm~$0.08 & 29.64&                &       \\
{NGC 6475}    & 
{29.32$~\pm~$0.07} & 
{29.25} & 
{29.45$~\pm~$0.05} & 
{29.44} &
              29.39$~\pm~$0.06 & 29.33 &
{--}      &
{--}      \\
\hline
\end{tabular}
\end{table}