\begin{table}%T3
%\centering
\par
\caption{Best-fit parameters of the power-law plus four Gaussians
model reproducing the [18$-$20]~\AA~ RGS-1 spectra of Mkn~421.
The error bars refer to the 90\% confidence intervals.}
\label{1820-tab}
\begin{tabular}{cccc}\hline\hline
\multicolumn{4}{c}{RGS-1 [18$-$20]~\AA}\\
\hline
Feature & $\lambda$ & {\it FWHM}  & $k$  \\
number  &  (\AA)    & (m\AA) & ( cts cm$^{-2}$ s$^{-1}$~\AA$^{-1}$)\\
\hline
\multicolumn{4}{c}{ToO observation}\\
\hline
2  & $18.661\pm0.004$ & $\rm 24^{+9}_{-6}$ & $-0.0040\pm0.0011$ \\
3  & $18.937\pm0.001$ & $8\pm1$ & $0.0111\pm0.0023$ \\
5  & $19.662\pm0.005$ & $30\pm10$ & $-0.0040\pm0.0010$ \\
6  & $19.890\pm0.001$ & $\rm 8.5^{+1.5}_{-1}$ & $-0.0098\pm0.0031$ \\
\hline
\multicolumn{4}{c}{Archival data}\\
\hline
2    & $18.683\pm0.001$ & $\rm 15.2^{+3}_{-2}$ & $-0.319\pm0.048$ \\
3    & $18.9585\pm0.001$ & $14.5\pm2$ & $-0.349\pm0.051$ \\
5    & $19.681\pm0.002$ & $15\pm3$ & $-0.266\pm0.053$ \\
6    & $19.910\pm0.002$ & $16\pm3$ & $-0.303\pm0.050$\\
\hline 
\multicolumn{4}{c}{Total}\\
\hline
2 & $18.680\pm0.004$ & $\rm 33^{+9}_{-7}$ & $-0.147\pm0.026$ \\
3 & $18.953\pm0.003$ & $\rm 28^{+5}_{-4}$ & $-0.177\pm0.029$ \\
5 & $19.676\pm0.003$ & $\rm 23^{+5}_{-4}$ & $-0.166\pm0.032$ \\
6 & $19.904\pm0.004$ & $\rm 33^{+8}_{-7}$ & $-0.128\pm0.027$ \\
\hline
\end{tabular}
\end{table}