\begin{table}%t7
%\centering
\par
\caption {\label{tab:o8vels}The mean wavelength and {\it FWHM} of the O~{\sc viii} line, grouping observations into two groups of two and maximizing the phase \mbox{sampling}.}
\begin{tabular}{lll} 
\hline \hline\\[-3mm]
Mean $\phi$ & $\lambda_{\rm o}$ $(-1)$ [\AA]   &  $\lambda_{\rm o}$ $(+1)$ [\AA]        \\  \hline 
0.05   & $18.976 \pm 0.010$ &  $18.970 \pm 0.010$   \\ 
0.27    & $18.971 \pm 0.009$ &  $18.977 \pm 0.006$  \\\hline
\\[-3mm]
 &  {\it FWHM} $(-1)$ [\AA]        & {\it FWHM} $(+1)$ [\AA]       \\  \hline 
0.05    &  $0.07 \pm 0.03$ & $0.06 \pm 0.03$ \\ 
0.27     &  $0.05 \pm 0.01$  & $0.05 \pm 0.02$ \\ 
 \hline
\end{tabular} 
\end{table}