\begin{table}%t1
\caption{Main properties of the  proper motion catalogues used in this
work. For  each catalogue we list: the 99\% magnitude completeness range
($\Delta  m$),  the  limiting  magnitude  ($m_{\rm lim}$),  the  reference
passband, the maximum time baseline  ($\Delta T$) in years, the reference epoch, the
absolute average  positional ($\sigma_{\alpha}$, $\sigma_{\delta}$) 
and proper  motion accuracies  ($\sigma \mu_{\alpha}$, $\sigma \mu_{\delta}$) in right ascension and declination, in mas and mas yr$^{-1}$, respectively.}
\label{pm_cat}
%\centerline
{
\par
\begin{tabular}{l|llllllccc|l}
\hline\hline
Catalogue & $\Delta m$ & $m_{\rm lim}$ & Band & $\Delta T$ & Epoch &  $\delta_{\alpha}$ & $\delta_{\delta}$ & $\sigma \mu_{\alpha}$ & $\sigma \mu_{\delta}$& Reference \\
 %         &            &           &      &            &        & (mas)             &  (mas)            &  (mas/yr)& (mas/yr)      &  \\
\hline
\hipp\        & 7.3--9  & 12.4  & $H_p$ & 1989.8--1993.2   & 1991.25  & 0.77 & 0.64 & 0.88 & 0.74 & Perryman et al. (1997) \\
\tycho\       & 1--11   & 15.2  & $V_T$ & 1989.8--1993.2   & 2000.00  & 60   & 60   & 2.5  & 2.5  & H{\o}g et al. (2000) \\
\ucac$^{1}$   &  n/a   & 16.0  & $R$   & 1998.1--2002.9   & 2000.00  & 15--70& 15--70& 1--7  & 1--7  & Zacharias et al. (2004)\\
\usnob        & 1--21   & 22.0  & $V_J$ & 1950--1998       & 2000.00  & 200  & 200  & $<$20&$<$20 & \cite{mo:03}\\
\gsc$^{2}$    & 1--20.5 & 22.5  & $F$   & 1950--1998       & 1987--1998& 250  & 250  & $<$10&$<$10 & in preparation \\ \hline
\end{tabular}}
\medskip
\par
$^1$ In its  present release, the \ucac\ catalogue  does not cover the
declination  strip  $+40^\circ  \le  \delta  \le  +52^\circ$  and  the
magnitude completeness is not uniform.
\\
$^2$ Version 2.3.2 (consortium  release). Proper motion information is
still incomplete and affected  by a 10 mas~yr$^{-1}$ systematic error
in  the southern  hemisphere ($\delta \le  -25^\circ$).   The positions  reported  in the  \gsc\
catalogue  have not  been normalized  to a  given reference  epoch but
refer to the mean epoch of the most recent observations.
\label{default}
\end{table}