\begin{table}%t8
\caption{\label{tab4}Summary of the multiplicity in young open clusters$^{{a}}$.}     
%\centerline
 {          
\begin{tabular}{c c c c c c c l}   
\hline\hline       
Cluster & Density  & Minimal binary  & n\# O-stars & Short-period  & Long-period  & Variable  & References\\
 & (stars/pc$^2$) & fraction & & binaries ($P$~$\le$~10) & binaries & stars\\
\hline                    
NGC~2244 & 2.84 & 17\% & 6 & 0 & 1 & 3 & This paper\\  
IC~1805 & 1.65 & 20\% & 10 & 1 & 1 & 4$^{\star}$ & \citet{rau04} \\
& & & & & & & \citet{deb06}\\
NGC~6231 & 12.46 & 63\% & 16 & 6$^{\star \star}$  & 4 & 3 & \citet{san08} \\  
\hline
\end{tabular}} 
\par
\smallskip
\par
$^{{a}}$ The first and second columns give the name and the density of the cluster. The third column provides the minimal binary fraction determined after intensive monitoring of the O-type stars. The next columns present the number of O-stars in each cluster, and the number of detected short and long-period binaries and the variable stars (potential binaries and probable intrinsic variables). The last column quotes relevant references. 
$^{\star }$~Notes: four late O-type stars were not studied by the two papers mentioned in the references. We will consider these objects as variable in the absence of a thorough study.
 $^{\star \star}$  Notes: we take into account that NGC~6231 also contains a WR$+$O binary since a WR is an evolved O-star.
\end{table}