  Recently, a class of Roche-lobe-filling binary systems consisting of hot
subdwarf stars and white dwarfs with sub-hour periods has been discovered. At
present, the hot subdwarf is in a shell He burning phase and is transferring
some of its remaining thin H envelope to its white dwarf companion. As the
evolution of the hot subdwarf continues, it is expected to detach, leaving
behind a low mass C/O core white dwarf secondary with a thick He layer. Then,
on a timescale of $\sim 10$ Myr, gravitational wave radiation will again bring
the systems into contact. If the mass transfer is unstable and results in a
merger and a catastrophic thermonuclear explosion is not triggered, it creates
a remnant with a C/O-dominated envelope, but one still rich enough in He to
support an R Corona Borealis-like shell burning phase. We present evolutionary
calculations of this phase and discuss its potential impact on the cooling of
the remnant white dwarf.
