\begin{table}%t4
\caption{Constraints on $\eta_{\rm HMXB}(\tau).$\label{tb:eta_hmxb}}
%\centerline
{
\begin{tabular}{lcccc}
\hline\hline
Name & Age, Myr &  $M_*^{(1)}$ & $N_{\rm HMXB}^{(2)}$& $\eta_{\rm HMXB}^{(3)}$\\
\hline
R136 & $\approx$$1{-}2$ & $2\times 10^4$ & $\le $2 & $\le$$ 1\times 10^{-4}$\\
LMC 4 & $\approx$$10{-}12$ & $5\times 10^4$ & $5$ & $(1\pm 0.45)\times 10^{-4}$\\
\hline
\end{tabular}}
\medskip
(1) -- Total mass of massive stars, $M\ge8~M_{\odot}$;
(2) -- number of HMXBs detected by XMM; 
(3) -- number of HMXBs per unit stellar mass; References for age and
IMF: R136 -- \citet{massey98} and \citet{sirianni00}, LMC 4 --
\citet{braun97}.
%, NGC1818 -- \citet{ngc1818} 
\end{table}