HST/COS Spectroscopy of H1504+65

We present new ultraviolet spectra of the peculiar white dwarf (WD) H1504+65, obtained with COS on HST. H1504+65 is the hottest known WD (Teff= 200,000 K) and has an atmosphere mainly composed of C and O, augmented with high amounts of Ne and Mg. This object is unique and the origin of its surface chemistry is completely unclear. We probably see the naked core of either a C--O WD or even a O--Ne--Mg WD. In the latter case, this would be the first direct proof that such WDs can be the outcome of single-star evolution. The new observations were performed to shed light on the origin of this mysterious object.