  The discrepancy between abundances computed using optical recombination lines
(ORLs) and collisionally excited lines (CELs) is a major, unresolved problem
with significant implications for the determination of chemical abundances
throughout the Universe. In planetary nebulae (PNe), the most common
explanation for the discrepancy is that two different gas phases coexist: a hot
component with standard metallicity, and a much colder plasma enhanced in heavy
elements. This dual nature is not predicted by mass loss theories, and direct
observational support for it is still weak. In this work, we present our recent
findings that demonstrate that the largest abundance discrepancies are
associated with close binary central stars. OSIRIS-GTC tunable filter imaging
of the faint O II ORLs and MUSE-VLT deep 2D spectrophotometry confirm that O II
ORL emission is more centrally concentrated than that of [O III] CELs and,
therefore, that the abundance discrepancy may be closely linked to binary
evolution.
