\begin{table}%t1
 \caption{Comparison of the Hipparcos \citep{1997AetA...323L..49P} and the PPM \citep{1994AetAS..105..301R} star catalogues.}\label{tab:CompStarCatalogues} 
\small%\centerline
 {
  \begin{tabular}{lcc}
  \hline \hline
  Catalogue & Hipparcos & PPM\\ 
  \hline
  No. of stars & 118 218 & 468 586 \\
  Stars/deg$^2$ & $\sim$3  &  $\sim$7.8 \\
  Mean err. position & 0.77/0.64$^a$ & 0.11/0.27$^b$\\
  Visual Mag. & 9.0 & 13.5 \\
\hline
 \end{tabular}}
 \medskip
 $^a$ $\sigma_{\alpha}$ $\sigma_{\delta}$ in milliarcsec, at J1991.25; 
 $^b$ $\sigma$ in arcsec, dependend on the hemisphere, at J1990.
\end{table}