\begin{table}%t9
  \caption{Intrinsic X-ray fluxes and luminosity in the 0.35--7.5 keV
    band ($f_{\rm X}$ and $L_{\rm X}$) as derived from the EPIC pn
    spectral fits for the X-ray-bright PMS 
    detected in L1551. Also given is the probability of constancy from
    the Kolmogorov-Smirnov (K--S) test for the \xmm\ EPIC-pn light
    curves (shown in Fig.~\ref{fig:spec}). The average flux level of
    the sources during the \xmm, ROSAT and ASCA observations (when
    available) in the same bands as used by Carkner et~al. (\cite{cfk+96}) is
    also reported. Carkner et~al. provide ROSAT and ASCA fluxes over
    different energy ranges for  each source; we  report these energy
    ranges in the columns "$\Delta E_{\rm ROSAT}$''  and "$\Delta
    E_{\rm ASCA}$'' and provide the source flux over the  same ranges
    derived from the fits to \xmm\ spectral data. Energy  ranges are
    in keV, flux units are $10^{-13}$ \ecms\ and X-ray  luminosity is in
    units of $10^{28}$ erg s$^{-1}$.} 
%%%\centerline
\par
{
  \begin{tabular}{ccccccccccc}
\hline  \hline
Source  & Name & $f_{\rm X}$ & $L_{\rm X}$ & K-S & $\Delta E_{\rm
  ROSAT}$ & $F_{\rm X}$\textsc{xmm} &
$F_x$\textsc{rosat} & $\Delta E_{\rm ASCA}$ & $F_{\rm X}$\textsc{xmm} &
$F_{\rm X} $\textsc{asca} \\
\hline
S2 &  MHO 5 & 0.66 & 15.6 & 0.79 & 0.2--2.0 & 0.34 & $\la$$0.4$ & 0.5--3.0 &
0.33 & -- \\
S3 & V826 Tau & 23.0 & 534 & $0.13\times10^{-4}$ & 0.2--2.0 & 12.9 &
15.0 & 0.5--3.0 & 13.4 & 15.8$^a$ \\
S6 & V827 Tau & 4.0 & 93.8 & $0.37\times10^{-1}$ & 0.2--2.0 & 1.9 & 13.0 &
0.8--3.5 & 1.5 & 4.5 \\
S7 & V1075 Tau & 8.5 & 199 & $0.2\times10^{-25}$ & 0.1--2.0 & 4.1 &
5.3 & 0.5--2.5 & 4.1 & 4.9 \\
S14 & V710 Tau & 5.5 & 131 & 0.35 & 0.2--2.0 & 2.2 & 6.9$^c$ & 0.5--3.0 &
2.5 & -- \\
S18 & XZ Tau & 55.3 & 1300 & $<$$10^{-38}$ & 0.5--2.0 & 2.9 & $\sim$$1.0$
& 0.5--2.0 & 2.9 & 15.0 \\
S19 & HL Tau & 10.0 & 235 & $0.74\times10^{-1}$ & 1.0--2.0$^f$ & 0.4 & $^d$ &
1.0--3.0 & 1.4 & $^d$ \\
S23 &  MHO~4 & 0.31 & 7.34 & 0.54 & 0.2--2.0 & 0.2 & 0.4$^c$ & 0.5--3.0 & 0.2
& -- \\
S25 &  MHO~9 & 0.24 & 5.55 & 0.59 & 0.2--2.0 & 0.2 & $\la$$0.4^e$ & 0.5--3.0 & 0.2
& -- \\ 
\hline
    \end{tabular}}
\medskip
  $^a$ Average flux, the source underwent a large flare during ASCA observations.\\
  $^c$ Computed using the approximate relation of Carkner et~al. (\cite{cfk+96}) 1 cts~${\rm (ks)^{-1}}
  \simeq 3 \times 10^{28}~{\rm ergs~s^{-1}}$.\\ 
  $^d$ Unresolved from XZ Tau.\\
  $^e$ As minimum flux level detectable by ROSAT we took the
  flux level of the weakest source detected by Carkner et~al. (\cite{cfk+96}), MHO 4.\\
  $^f$ In our data HL Tau was clearly visible in the image only for
  energies $\Delta E > 1.0$~keV (see text).
    \label{tab:protovar}
\end{table}