H 1504 $ + $ 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 H 1504 $ + $ 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 .