  We exploit the recent discovery of pulsations in mixed-atmosphere (He/H),
extremely low-mass white dwarf precursors (ELM proto-WDs) to test the
proposition that rotational mixing is a fundamental process in the formation
and evolution of low-mass helium core white dwarfs. Rotational mixing has been
shown to be a mechanism able to compete efficiently against gravitational
settling, thus accounting naturally for the presence of He, as well as traces
of metals such as Mg and Ca, typically found in the atmospheres of ELM
proto-WDs. Here we investigate whether rotational mixing can maintain a
sufficient amount of He in the deeper driving region of the star, such that it
can fuel, through HeII-HeIII ionization, the observed pulsations in this type
of stars. Using state-of-the-art evolutionary models computed with MESA, we
show that rotational mixing can indeed explain qualitatively the very existence
and general properties of the known pulsating, mixed-atmosphere ELM proto-WDs.
Moreover, such objects are very likely to pulsate again during their final WD
cooling phase.
