  V605 Aquilae is today widely assumed to have been the result of a final
helium shell flash occurring on a single post-asymptotic giant branch star. The
fact that the outbursting star is in the middle of an old planetary nebula and
that the ejecta associated with the outburst is hydrogen deficient supports
this diagnosis. However, the material ejected during that outburst is also
extremely neon rich, suggesting that it derives from an oxygen-neon-magnesium
star, as is the case in the so-called neon novae. We have therefore attempted
to construct a scenario that explains all the observations of the nebula and
its central star, including the ejecta abundances. We find two scenarios that
have the potential to explain the observations, although neither is a perfect
match. The first scenario invokes the merger of a main sequence star and a
massive oxygen-neon-magnesium white dwarf. The second invokes an
oxygen-neon-magnesium classical nova that takes place shortly after a final
helium shell flash. The main drawback of the first scenario is the inability to
determine whether the ejecta would have the observed composition and whether a
merger could result in the observed hydrogen-deficient stellar abundances
observed in the star today. The second scenario is based on better understood
physics, but, through a population synthesis technique, we determine that its
frequency of occurrence should be very low and possibly lower than what is
implied by the number of observed systems. While we could not envisage a
scenario that naturally explains this object, this is the second final flash
star which, upon closer scrutiny, is found to have hydrogen-deficient ejecta
with abnormally high neon abundances. These findings are in stark contrast with
the predictions of the final helium shell flash and beg for an alternative
explanation.
