\begin{table}%t4
\caption{\label{om}Optical-UV magnitudes of BL Lacertae derived from the data analysis of the OM~frames.}      
%\centerline
 {
\begin{tabular}{c c c c c c c}  
\hline\hline            
Date & $V$ & $B$ & $U$ & UV$W1$ & UV$M2$ & UV$W2$ \\     
\hline                                   
\multicolumn{7}{c}{Exposure times (s)}\\
July 10--11, 2007 &  1498 & 1499 & 1499 & 2601 & 3098 & 6257$^a$ \\
December 5, 2007  &  2100 & 2099 & 2099 & 3500 & 3780 & 4000 \\
January 8, 2008   &  1700 & 1700 & 1698 & 2799 & 3300 & 7359$^b$   \\
\hline
\multicolumn{7}{c}{Magnitudes}\\
July 10--11, 2007 &  $15.33 \pm 0.10$ & $16.27 \pm 0.10$ & $15.87 \pm 0.10$ & $15.99 \pm 0.10$ & $16.90 \pm 0.10$ & $17.13 \pm 0.14$\\
December 5, 2007  &  $14.96 \pm 0.10$ & $15.91 \pm 0.10$ & $15.52 \pm 0.10$ & $15.62 \pm 0.10$ & $16.45 \pm 0.10$ & $16.98 \pm 0.11$\\
January 8, 2008   &  $14.97 \pm 0.10$ & $15.90 \pm 0.10$ & $15.53 \pm 0.10$ & $15.65 \pm 0.10$ & $16.47 \pm 0.10$ & $16.88 \pm 0.14$\\
\hline
\end{tabular}}
\par
 \smallskip
{$^a$ Two exposures of 3579 and 2678 s.}\\
{$^b$ Two exposures of 3779 and 3580 s.}\\
\par
\end{table}