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We report the identification of the double degenerate system NLTT 16249 thatcomprises a normal, hydrogen-rich (DA) white dwarf and a peculiar,carbon-polluted white dwarf (DQ) showing photospheric traces of nitrogen. Wedisentangled the observed spectra and constrained the properties of bothstellar components. In the evolutionary scenario commonly applied to thesequence of DQ white dwarfs, both carbon and nitrogen would be dredged up fromthe core. The C/N abundance ratio (~ 50) in the atmosphere of this unique DQwhite dwarf suggests the presence of unprocessed material (14N) in the core orin the envelope. Helium burning in the DQ progenitor may have terminated earlyon the red-giant branch after a mass-ejection event leaving unprocessedmaterial in the core although current mass estimates do not favor the presenceof a low-mass helium core. Alternatively, some nitrogen in the envelope mayhave survived an abridged helium-core burning phase prior to climbing theasymptotic giant-branch. Based on available data, we estimate a relativelyshort orbital period (P <~ 13 hrs) and on-going spectroscopic observations willhelp determine precise orbital parameters.

The Core Composition of a White Dwarf in a Close Double Degenerate System