\begin{table}%t1
%%\centering
% \begin{minipage}{140mm}
\caption{ The values$^a$ of $T_{0,v}$ and $t^2_v$.
 \label{tab1}}
%\centerline
 {  \begin{tabular}{@{}cccc@{}}
\hline\hline
Object & {IC~2501} & {IC~4191} & {NGC~2440} \\
\hline
$T_0$ (K)     & 7700 &  8600 & 12~600 \\
$t^2$         & 0.053&  0.040& 0.077 \\
$T_{0,v}$ (K) & 11~500&  10~200& 14800 \\
$t^2_v$       & 0.002&  0.001& 0.011 \\
\hline
\end{tabular}}
\par
\smallskip
$^{a}$ For the low-excitation PN, IC~2501, $T_{0,v}$ and $t^2_v$
are deduced using the [N~{\sc ii}] $\lambda\lambda5754,~6678$~lines; for
the high-excitation PNe, IC~4191 and NGC~2440, $T_{0,v}$ and $t^2_v$
are deduced using the [O~{\sc iii}]~$\lambda\lambda4363,~5007$~lines.
\end{table}