  The post-Asymptotic Giant Branch (AGB) star HR4049 is in an eccentric binary
system with a relatively short period probably surrounded by a dusty
circumbinary disk. Extremely anomalous oxygen isotopic ratios, O16/O17 ~
O16/O18 ~ 7, have been measured from CO_2 molecules likely residing in the
disk. Such a composition cannot be explained in the framework of AGB and
post-AGB evolution while it can be qualitatively associated with the
nucleosynthesis occurring during nova outbursts. We discuss nova models, the
presence of a white dwarf companion to HR4049 and possible scenarios for the
dynamical evolution of this binary system. Circumbinary disks in which mixing
occurs between red-giant and nova material may also be invoked as the site of
formation of some rare types of meteoritic presolar grains.
