  H1504+65 is a young white dwarf with an effective temperature of 200,000 K
and is the hottest post-AGB star ever analysed with detailed model atmospheres.
Chandra LETG+HRC-S spectra have revealed the richest X-ray absorption line
spectrum recorded from a stellar photosphere to date. The line forming regions
in this extremely hot photosphere produce many transitions in absorption that
are also observed in emission in cool star coronae. We have performed a
detailed comparison of Chandra spectra of H1504+65 with those of Procyon and
alpha Cen A and B. State of the art non-LTE model spectra for the hot white
dwarf have enabled us to identify a wealth of absorption lines from highly
ionized O, Ne and Mg. In turn, these features have allowed us to identify
coronal lines whose origins were hitherto unknown.
