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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 amountsof Ne and Mg. This object is unique and the origin of its surface chemistry iscompletely unclear. We probably see the naked core of either a C--O WD or evena O--Ne--Mg WD. In the latter case, this would be the first direct proof thatsuch WDs can be the outcome of single-star evolution. The new observations wereperformed to shed light on the origin of this mysterious object.

HST/COS Spectroscopy of H1504+65