\begin{table} 
\caption{\label{tab3}The data for the USNO PN sample.} 
%\centerline
\par
\begin{tabular}{cccc}
\hline \hline 
Name & Trigonometric parallax &  $S_{\rm 5~GHz}$  &  Diameter  \\
 & [mas] & [mJy] & [$^{\prime\prime}$] \\ \noalign{\smallskip}
\hline
\noalign{\smallskip}
NGC 7293 (036.1-57.1) & $4.56\pm0.49$ & 1292$^a$ & 660$^b$ \\
NGC 6853 (060.8-03.6) & $3.81\pm0.47$ & 1325$^a$ & 340$^a$ \\
NGC 6720 (063.1+13.9) & $1.42\pm0.55$ & \p0384$^a$  & \p060$^c$\\
A 21 (205.1+14.2)     & $1.85\pm0.51$ & \p0157$^d$  & 550$^e$ \\
A 7 (215.5-30.8)      & $1.48\pm0.42$ & \p0305$^a$  & 760$^a$ \\
A 24 (217.1+14.7)     & $1.92\pm0.34$ & \p0\p036$^a$   & 415$^d$ \\
\hline
\end{tabular} 
\medskip
References: \\
$^a$ \cite{Cahn92};  
$^b$ \cite{1998AJ....116.1346O};\\
 $^c$~\cite{1974AA....35..219G};  
$^d$ \cite{1999AAS..138..275H}; \\$^e$ \cite{1984AA...137..291S}.
\end{table}