\begin{table}%t6
%\centering
\par
\caption{\label{spvel} Space velocities of the studied associations: mean value 
and standard deviation~($\sigma$) resulting from a Gaussian model, in units of~km~s$^{-1}$.\label{tab6}}
\scriptsize{
\begin{tabular}{lrcrcrcrcrcrc}
\hline \hline \\[-3pt]
Association & \multicolumn{2}{c}{$U$} & \multicolumn{2}{c}{$V$} & \multicolumn{2}{c}{$W$} & \multicolumn{2}{c}{$U_{\rm LSR}$} & \multicolumn{2}{c}{$V_{\rm LSR}$} & \multicolumn{2}{c}{$W_{\rm LSR}$} \\
             & {\tiny mean} & {\tiny $\sigma$} & {\tiny mean} & {\tiny $\sigma$} & {\tiny mean} & {\tiny $\sigma$} & {\tiny mean} & {\tiny $\sigma$} & {\tiny mean} & {\tiny $\sigma$} & {\tiny mean} & {\tiny $\sigma$} \\[3pt]
\hline 
US  & $-6.7$ & {\tiny 5.9} & $-16.0$ & {\tiny 3.5} & $-8.0$ & {\tiny 2.7} & 2.4 & {\tiny 5.0} & $-10.0$ & {\tiny 3.3} & $-0.9$ & {\tiny 2.8} \\
UCL &$ -6.8$ & {\tiny 4.6} & $-19.3$ & {\tiny 4.7} & $-5.7$ & {\tiny 2.5} & 3.2 & {\tiny 4.2} & $-14.4$ & {\tiny 4.6} & 1.4  & {\tiny 2.4} \\
LCC & $-8.2$ & {\tiny 5.1} & $-18.6$ & {\tiny 7.3} & $-6.4$ & {\tiny 2.6} & 1.1 & {\tiny 5.8} & $-13.6$ & {\tiny 5.6} & 1.1  & {\tiny 2.4} \\
Cha$^{\rm a}$ & $-11.9$ & {\tiny 7.7} & $-16.0$ & {\tiny 11.4} & $-8.6$ & {\tiny 5.8} & $-1.9$ & {\tiny 7.7} & $-10.8$ & {\tiny 11.4} & $-1.4$ & {\tiny 5.8} \\
\hline
\end{tabular}}
\begin{flushleft}
$^{\rm a}$ For the Cha OB association the mean velocities and the $\sigma$ 
presented are simple statistics, not derived from Gaussian fits.
\end{flushleft}
\par
\end{table}