\begin{table}%T4
%\centering
\par
\caption{The periods found from the period analysis of the mean $V$~magnitude
         and the peak-to-peak $V$ amplitude variations between different
         observing seasons. The periods on the left are those found by the
         method of Horne \& Baliunas (\cite{hb86}), and on the right are the periods found by
         PDM (Stellingwerf \cite{stellingwerf78}).}
\label{tab:periods}
\begin{tabular}{llll}
\hline\hline
\multicolumn{2}{c}{HB}&\multicolumn{2}{c}{PDM}\\
Mean&Amplitude&Mean&Amplitude\\
\hline
24.6 yr&--&24.4 yr&27.0 yr\\
--&13.0 yr&--&11.4 yr\\
\hline
\end{tabular}
\end{table}