\begin{table}%T2
\par
\caption{Weighted mean linear polarization parameters of HD~31648.
          $N$ is the number of individual measurements in each data point,
$\sigma_{\rm p}$ and~$\sigma_{\theta}$ are the estimated errors of the
weighted mean values of the polarization degree and of the
position angle, respectively.} \label{t2}
%\centering
\par
\begin{tabular}{ccccccc|ccccccc}
\hline\hline
 Date/& $N$ & Band &$P$  & $\sigma_{\rm p}$   & $\theta$\degr & $\sigma_{\theta}\degr$
&Date/& $N$ & Band & $P$  & $\sigma_{\rm p}$ & $\theta$\degr & $\sigma_{\theta}\degr$\\
JD~2~400~000+ &   &   & \%   &   \%       &                &                  &
JD~2~400~000+  &  &    & \%   &   \%       &                &                  \\
\hline
01.11.96  & 40  &   $U$  & 0.094 & 0.014 &14.3& 5.1&  07.11.96& 18  & $U$  & 0.198 &  0.020 & 32.9&  3.6   \\
50~389.39&     &  $ B$  & 0.205 & 0.009 &33.3& 1.7&50~395.33&     & $B$  & 0.192 &  0.012 & 53.9&  2.2 \\
          &     &   $V$  & 0.243 & 0.012 &39.6& 1.8&          &     & $V$  & 0.312 &  0.017 & 49.5&  2.4      \\
          &     &   $R$  & 0.305 & 0.009 &42.6& 0.9&          &     & $R$  & 0.310 &  0.012 & 51.4&  1.2      \\
          &     &   $I$  & 0.317 & 0.012 &46.4& 1.2&          &     & $I$  & 0.334 &  0.017 & 52.0&  1.4      \\
\hline
  01.11.96& 49  &   $U$  & 0.134 &   0.013&  25.3 & 3.8&  07.11.96& 8   & $U$  & 0.108 &  0.025 & 48.3 & 8.1   \\
50~389.58&     &   $B$  & 0.209 &   0.008&  39.4 & 1.7&50~395.62&     & $B$  & 0.220 &  0.015 & 51.9 & 2.5 \\
          &     &   $V$  & 0.252 &   0.011& 37.7 & 1.6&          &     & $V$  & 0.280 &  0.021 & 49.3 & 2.4      \\
          &     &   $R$  & 0.324 &   0.008& 41.6 & 1.0&          &     & $R$  & 0.337 &  0.015 & 51.1 & 1.3      \\
          &     &   $I$  & 0.358 &   0.011& 44.0 & 1.0&          &     & $I$  & 0.381 &  0.021 & 57.0 & 1.5      \\
\par
\hline
  04.11.96& 24  &   $U$  & 0.212 &   0.019&  19.3& 3.4&  08.11.96& 25   & $U$  & 0.126 &  0.16&  39.5& 5.8   \\
50~392.33&     &  $ B$  & 0.233 &   0.012&  41.7& 1.5&50~396.46&     & $B$  & 0.198 &  0.010&  50.6&  2.2 \\
          &     &   $V$  & 0.264 &   0.015&  44.4& 1.9&          &     & $V$  & 0.236 &  0.014&  48.6&  2.0      \\
          &     &   $R$  & 0.309 &   0.011&   45.4& 1.5&          &     & $R$  & 0.321 &  0.011&  53.4&  1.4      \\
          &     &   $I$  & 0.339 &   0.015& 48.0&  1.4&          &     & $I$  & 0.325 &  0.014&  57.1&  1.8      \\
\hline  
04.11.96& 32   &   $U$  & 0.073 &  0.014&  22.4& 6.3&  09.11.96& 52  & $U$  & 0.092&   0.018&  36.0&  8.9   \\
50~392.41&     &   $B$  & 0.220 &  0.009&  45.4& 1.7&50~397.37&     & $B$  & 0.181&   0.011&  59.2&  3.2 \\
          &     &   $V$  & 0.227 &  0.012&  45.9& 1.9&       &        & $V$  & 0.187&   0.014&  62.4&  3.7      \\
          &     &   $R$  & 0.336 &  0.009&  47.1& 1.3&       &        & $R$  & 0.282&   0.011&  59.8&  2.4      \\
          &     &   $I$  & 0.381 &  0.013&  49.8& 1.4&       &        & $I$  & 0.371&   0.015&  58.9&  1.8      \\
\hline
04.11.96& 16  &   $U$  & 0.175 &  0.021&  26.6 & 4.5&  28.03.98& 8   & $U$    & 0.238&   0.044&  14.4 & 8.1   \\
50~392.62&     &   $B$  & 0.197 &  0.013&  48.7 & 2.9&50~901.31&     &   $B$  & 0.254&   0.025&  37.1 & 4.2 \\
          &     &   $V$  & 0.247 &  0.018& 47.0 &2.2&               &  & $V$  & 0.358&   0.031&  48.4 & 3.0      \\
          &     &   $R$  & 0.299 &  0.013& 50.1 & 2.0&       &          & $R$  & 0.336&   0.031&  50.8 & 2.2      \\
          &     &   $I$  & 0.376 &  0.018& 48.7 & 1.8&       &  & $I$          & 0.372&   0.029&  37.7 & 3.4      \\
\hline
  05.11.96& 23  &   $U$  & 0.195&    0.016&  35.9 & 3.4&  30.03.98& 14   & $U$  & 0.337 &  0.030 & 34.7&  7.7   \\
50~393.50&     &   $B$  & 0.186&    0.010&  47.0 & 1.9&50~903.27&     &   $B$& 0.288 &  0.017 & 35.1&  3.8 \\
          &     &   $V$  & 0.265&    0.014&  49.4 & 1.6&          &  & $V$     & 0.308 &  0.021 & 42.5&  2.3      \\
          &     &   $R$  & 0.327&    0.011&  50.9 & 1.3&          &  & $R$     & 0.297 &  0.016 & 43.3&  2.3      \\
          &     &   $I$  & 0.360&    0.015& 52.5 & 1.2&          &  & $I$     & 0.309 &  0.020 & 43.8& 2.5      \\
\hline
  05.11.96& 36   &   $U$  & 0.111 &   0.013 & 37.2 & 4.5&  31.03.98& 9   & $U$  & 0.119 &  0.048&  42.3&  14.2   \\
50~393.59&     &   $B$  & 0.229 &   0.008 & 47.8 & 1.5&50~904.26&     &   $B$& 0.308 &  0.025&  43.1&  3.3 \\
          &     &   $V$  & 0.281 &   0.011 & 49.0 & 1.5&          &  & $V$     & 0.245 &  0.030&  44.2&  6.1      \\
          &     &   $R$  & 0.343 &   0.009 & 53.6 & 1.0&          &  & $R$     & 0.395 &  0.021&  49.3&  4.3      \\
          &     &   $I$  & 0.396 &   0.012 &  53.4 & 1.0&          &  & $I$     & 0.531 &  0.028&  51.4&  3.2      \\
\hline
  06.11.96& 12   &   $U$  & 0.233 &   0.037&  48.7&  4.9& 01.04.98  & 8 & $U$ & 0.100 & 0.033 & 30.4 & 14.2   \\
50~394.32&     &   $B$  & 0.244 &   0.022&  49.1&  3.6& 50~905.25    &  & $B$ & 0.277 & 0.020 & 41.3 & 3.4  \\
          &     &   $V$  & 0.188 &   0.029&  35.1& 5.0&          &  & $V$ & 0.278 & 0.026 & 43.1 & 2.3   \\
          &     &  $ R$  & 0.334 &   0.022&  55.5&  2.2&          &  & $R$ & 0.343 & 0.019 & 52.2 & 2.2   \\
          &     &   $I$  & 0.354 &   0.030& 48.3& 3.0&          &  & $I$ & 0.381 & 0.026 & 51.1 & 2.2   \\
\hline
\end{tabular}
\end{table}