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