\begin{table}%t1
  \caption{\label{table:results}Polarimetric results.}
  %\centerline
{ 
  \begin{tabular}{l@{~~}c@{~~}c@{~~}c@{~~}c}
    \hline\hline
              & ALFOSC & ALFOSC & ALFOSC & FORS~1 \\
              & set 1  & set 2  & set 3  & sum \\
     $\Delta t$ (days) & 0.37 & 0.38 & 0.40 & 0.67 \\
    \hline
     \multicolumn{5}{l}{\textit{no ISM correction}} \\
     $P$ (\%)       & $1.17 \pm 0.46$     & $1.73 \pm 0.51$    & $0.15 \pm 0.49$     & $1.29 \pm 0.13$ \\
     $\theta$ (\degr) & $184.2 \pm 11.4$ $^{{\rm I}}$ & $166.4 \pm 8.1$ & $129.8 \pm 41.0$ $^{{\rm II}}$ & $121.8 \pm 2.8$ \\
     \multicolumn{5}{l}{\textit{with ISM correction}} \\
     $P$ (\%)      & $1.72 \pm 0.56$     & $2.09 \pm 0.60$    & $<$0.59        & $0.75 \pm 0.42$ \\
     $\theta$ (\degr) & $ 187.7 \pm 8.3$ $^{{\rm I}}$  & $173.0 \pm 7.9$ & $-$                     & $132.5 \pm 13.9$ \\
    \hline
  \end{tabular}}
  \smallskip
      $^{{\rm I}}$ We added 180 to the value of the angle for clarity.\\
   $^{{\rm II}}$ The very low value for the degree of linear polarization makes the value for position angle very insecure, which is reflected in the large error. See also the text for comments on this data point.
\end{table}