\begin{table}%t1
\caption{\label{tab_obs}Observational data of the program stars.}
\small%\centerline
 {
\begin{tabular}{lcccccccc}
\hline
\hline
        &                &          & \multicolumn{2}{c}{Far-UV} & \multicolumn{2}{c}{UV} & \multicolumn{2}{c}{Optical}  \\
star & spectral type$^{{a}}$ &  $V$ & data set$^{b}$ & date & data set & date & instrument & date  \\
\hline
  HD~216898  & O9IV, O8.5V & 8.04 & F-A0510303 & 2000-08-03 & SWP43934 & 1992-02-05 & OHP/ELODIE & 2004-08-29      \\
  HD~326329  & O9V         & 8.76 & F-B0250501 & 2001-08-09 & SWP48698 & 1993-09-21 & ESO/FEROS  & 2005-06-25      \\
  HD~66788   & O8V, O9V    & 9.43 & F-P1011801 & 2000-04-06 & SWP49080 & 1993-11-03 & ESO/FEROS  & 2004-12-22 \\
 $\zeta$~Oph & O9.5Vnn     & 2.56 & C-C002     & 1972-08-29$^{{\dagger}}$ & SWP36162 & 1989-04-29 & OHP/ELODIE & 1998-06-11  \\
  HD~216532  & O8.5V((n))  & 8.03 & F-A0510202 & 2000-08-02 & SWP34226 & 1988-09-11 & OHP/ELODIE & 2005-11-10 \\
  \hline
\end{tabular}}
\medskip
$^{{a}}$ Spectral types come from Lesh (\cite{Lesh68}), Schild et~al. (\cite{Schi69}), Garrison (\cite{Garr70}), Walborn (\cite{Walb73}), and MacConnell \& Bidelman (\cite{Macc76}). \\
$^{{b}}$  Satellite used to obtain spectra: F~= FUSE and C~= Copernicus. \\
$^{{\dagger}}$ Orbits \#115-247 (Snow et~al. 1977).
\end{table}