\begin{table}%t1
\caption{\label{tab:obslog}Summary of LETGS observations used in this paper.}
%\centerline
{\small
\begin{tabular}{lcccc}
\hline\hline
Source                & Sirius~B         & Sirius~B         & Sirius~B  & HZ~43A \\
ObsID                 & 1421             & 1452             & 1459      & 59 \\
\hline
start date            & 28 Oct. 1999      & 26 Oct. 1999      & 27 Oct. 1999      & 12 Nov. 1999 \\
Duration (ks)         & 25.3             & 28.0             & 12.0             & 40.2 \\
Net exposure (ks)$^a$ & 13.8             & 19.0             & 11.8             & 38.9 \\
0th order counts/s    & $1.282\pm 0.010$ & $1.249\pm 0.008$ & $1.310\pm 0.011$ & $8.826\pm 0.015$ \\
PHA difference$^b$    & $-5.6\pm 2.0$    & $-16.2\pm 1.9$   & $-6.5\pm 2.0$    & $-7.0\pm 1.4$ \\
\hline
\end{tabular}}
\smallskip
\par
$^a$ Dead-time corrected exposure time after filtering out periods of high
background.
$^b$ The PHA (pulseheight) difference is the average PHA of the spectrum
relative to a standard spectrum.
\end{table}