\begin{table}%T6
%\centering
\par
\caption{Summary of absorption features observed in the RGS spectrum of Mkn~421.
$^a$ Archival data analyzed by us. 
$^b$ Detected in one of the two observations.
C01: Cagnoni (\cite{Cagnoni2001}); N01: Nicastro et~al. (\cite{Nicastro2001}); P01: Paerels et~al. (\cite{Paerels2001});
N02: Nicastro et~al. (\cite{Nicastro2002}); N03: Nicastro et~al. (\cite{Nicastro2003});
dV03: de~Vries (\cite{Vries2003}); F03: Fang et~al. (\cite{Fang2003}); C04: Cagnoni et~al. (\cite{Cagnoni2004}).}
\label{tab3}
\begin{tabular}{c|c|c|c|c|c}
\hline\hline
Feature & RGS-1           & RGS-2   & Crab  & Literature & Other \\
number & $\lambda$ (\AA)  &          &       &            & sources \\
\hline
2 & $18.680 \pm 0.004$ & $18.697$ (ToO)     & $\rm 18.674^{+0.002}_{-0.003}$ & YES (C01--XMM) & PKS~2155-304 (N02--Cha)\\
  &                    & $18.718$ (4/11/02) &                            &  NO (N01--Cha) & PKS~2155-304 (C04--XMM)\\
  &                    & instrumental       &                            & YES (N03--Cha) & \\
\hline
3 & $18.953 \pm 0.003$ & NO                 & instrumental               & YES (C01--XMM) & PKS~2155-304 (N02--Cha)\\
  &                    &                    &                            &  NO (N01--Cha) & 3C 273 (F03--Cha)\\
  &                    &                    &                            & YES (N03--Cha) & NO PKS~2155-304 (C04--XMM)\\
\hline
5 & $19.676 \pm 0.003$ & NO                 & NO                         &  NO            & NO \\
\hline
6 & $19.904 \pm 0.004$ & instrumental       & instrumental               &  NO            & PKS~2155-304 (C04--XMM)\\
  &                    &                    &                            &                & NO PKS~2155-304 (N02--Cha)\\
\hline
7 & $21.603 \pm 0.008$ & no data            & NO                         & YES (C01--XMM) & PKS~2155-304 (N02--Cha)\\
  &                    &                    &                            &  NO (N01--Cha) & PKS~2155-304 (C04--XMM)\\
  &                    &                    &                            & YES (N03--Cha) & 3C 273 (F03--Cha)\\
\hline
8 & $21.823\pm0.001$   & no data            & instrumental               & YES (C01--XMM) & PKS~2155-304$^a$ (XMM)\\
  &     	       &                    &                            & YES$^b$ (N01--Cha) & NO PKS~2155-304 (C04--XMM) \\
  &                    &                    &                            & YES (N03--Cha) & \\		
 % &                    &                    &                            &                & \\
\hline
9 & $22.778\pm0.001$   & no data            & NO                         & YES (dV03--XMM)& PKS~2155-304 (dV03--XMM)\\
  &                    &                    & near                       &                & PKS~2155-304 (dV03--Cha)\\
  &       	       &                    & instrumental               &                & PKS~2155-304 (C04--XMM)\\
  &                    &                    & feature                    &                & NO ScoX1(dV03--XMM) \\
  &                    &                    &                            &                & NO 4U 0614+91 (dV03--XMM)\\
\hline       
10 & $23.510\pm0.007$  & no data            &                            & YES (dV03--XMM)& PKS~2155-304 (P01--Cha) \\
   &                   &                    &                            &                & PKS~2155-304 (N02--Cha)\\
   &                   &                    &                            &                & PKS~2155-304 (C04--XMM)\\ 
\hline
\end{tabular}
\end{table}