\begin{table}%t3
 %\centering
\par
%\caption {Number of maser sources in the sample depending on 
%the presence ($+$) and absence ($-$) of four OH transitions.}
\caption{Number of maser sources in the sample depending on 
the presence ($+$) and absence ($-$) of four OH transitions.}\label{t3}
\begin{tabular}{c c c c c c c c c}
\hline
\hline
1612 & & $+$ & $+$ & $+$ & $-$ & $-$ & $-$ & $-$ \\
1665 & & $-$ & $+$ & $+$ & $+$ & $+$ & $-$ & $+$ \\
1667 & & $-$ & $-$ & $+$ & $-$ & $+$ & $+$ & $+$ \\
1720 & & $-$ & $-$ & $-$ & $-$ & $-$ & $-$ & $+$ \\
\hline
Frequency &  &  1  &  3  &  1  &  11 &  32 &  1  &  6  \\
 (MHz)    &  & \multicolumn{7}{l}{Number of sources}              \\
\hline
\end{tabular}
\end{table}