  Abell 70 (PN G038.1-25.4, hereafter A 70) is a planetary nebula (PN) known
for its diamond ring appearance due a superposition with a background galaxy.
The previously unstudied central star is found to be a binary consisting of a
G8IV-V secondary at optical wavelengths and a hot white dwarf (WD) at UV
wavelengths. The secondary shows Ba II and Sr II features enhanced for its
spectral type that, combined with the chromospheric Halpha emission and
possible 20-30 km/s radial velocity amplitude, firmly classifies the binary as
a Barium star. The proposed origin of Barium stars is intimately linked to PNe
whereby wind accretion pollutes the companion with dredged-up material rich in
carbon and s-process elements when the primary is experiencing thermal pulses
on the Asymptotic Giant Branch (AGB). A 70 provides further evidence for this
scenario together with the other very few examples of Barium central stars. The
nebula is found to have Type-I chemical abundances with helium and nitrogen
enrichment, which when combined with future abundance studies of the central
star, will establish A 70 as a unique laboratory for studying s-process AGB
nucleosynthesis. We also discuss guidelines to discover more binary central
stars with cool secondaries in large orbits that are required to balance our
knowledge of binarity in PNe against the currently better studied post
common-envelope binary central stars.
