\begin{table}%t4
  \caption{\label{tb4}Rotational offsets of Hipparcos and FK5~proper motions
required to constrain the comparison of the two proper motion systems to
the VLBI-determined correction of the luni-solar precession,
$\Delta p=-3.0$~mas/yr.}
%\centering
\par
 \begin{tabular}{lclclcl}   
   \hline \hline    
   ~ & ~ & \multicolumn{1}{c}{[mas/yr]} & ~ & \multicolumn{1}{c}{[mas/yr]} & ~ & \multicolumn{1}{c}{[mas/yr]}   \\   
   \hline     
$\dot{R}$:                & ~ & $\omega_x\hspace{9.80mm}=-0.30$         & ~ &
                                  $\omega_y\hspace{9.80mm}=0.60$          & ~ & $\omega_z\hspace{9.80mm}=~~~0.70$     \\
$\Delta\dot{R}$~:          & ~ & $\Delta\omega_x\hspace{7.8mm}=~~~0.06$ & ~ &
                                  $\Delta\omega_y\hspace{7.8mm}=0.12$     & ~ & $\Delta\omega_z\hspace{7.8mm}=-0.71$   \\
$\Delta\dot{R}(\rm FK5)$~: & ~ & $\Delta\omega_x(\rm FK5)=~~~0.24$      & ~ &
                                  $\Delta\omega_y(\rm FK5)=0.48$          & ~ & $\Delta\omega_z(\rm FK5)=-1.10$        \\  
   \hline   
   \end{tabular}
 \end{table}