  Classical novae are powered by thermonuclear runaways that occur on the white
dwarf component of close binary systems. During these violent stellar events,
whose energy release is only exceeded by gamma-ray bursts and supernova
explosions, about 10-4 10-5 Msun of material is ejected into the interstellar
medium. Because of the high peak temperatures attained during the explosion,
Tpeak ~ (1-4)x10+8 K, the ejecta are enriched in nuclear-processed material
relative to solar abundances, containing significant amounts of 13C, 15N, and
17O and traces of other isotopes. The origin of these metal enhancements
observed in the ejecta is not wellknown and has puzzled theoreticians for about
40 years. In this paper, we present new 2-D simulations of mixing at the
core-envelope interface. We show that Kelvin-Helmholtz instabilities can
naturally lead to self-enrichment of the solar-like accreted envelopes with
material from the outermost layers of the underlying white dwarf core, at
levels that agree with observations.
