  We investigate the evolutionary properties of AM~CVn stars with massive white
dwarf donors and accretors. As a representative of them we consider a binary
initially composed by a 0.30 M$_\odot$, He WD and a 1.02 M$_\odot$, CO WD. We
evaluate the time-dependent mass transfer rate from the donor and compute the
evolution of the accretor, accounting for the effects of mass exchange on the
evolution of orbital parameters. We model the thermal response of the accreting
CO WD with the FUN evolutionary code coupled to a full nuclear network, from H
to Bi, including more than 700 isotopes linked by about 1000 nuclear processes.
We find that accretors in these systems evolve through the stages of steady
He-burning and mild and strong He-flashes and become at the end CO WDs capped
by a massive ($\sim 0.1$ M$_\odot$) He-rich buffer. During He-flashes (both
mild and strong) the temperature in the He-shell increases above $3\times 10^8$
K, so that the ${^{22}Ne}(\alpha,n){^{25}Mg}$ reaction becomes efficient and
$n$-rich isotopes can be produced. During the RLOF episodes triggered by strong
non-dynamical He-flashes matter enriched in $\alpha$-elements and $n$-rich
isotopes is ejected, polluting the interstellar medium. Our results strongly
suggest that massive AM CVn systems with WD donors do not experience a final
very strong dynamical He-flash driving an explosive event like SN .Ia. Though
the ejected matter is highly enriched in heavy isotopes, the relative
contribution of massive AM CVn systems to the Galactic chemical evolution is,
most probably, negligible due to their expected paucity.
