  Post-asymptotic giant branch stars (post-AGB) in binary systems, with typical
orbital periods between ~100 to ~1000 days, result from a poorly understood
interaction that terminates their precursory AGB phase. The majority of these
binaries display a photospheric anomaly called 'chemical depletion', thought to
arise from an interaction between the circumbinary disc and the post-AGB star,
leading to the reaccretion of pure gas onto the star, devoid of refractory
elements due to dust formation. In this paper, we focus on a subset of
chemically peculiar binary post-AGBs in the Galaxy and the Magellanic Clouds
(MCs) whose high-resolution optical spectroscopic study revealed a carbon and
s-process enrichment, contrary to the commonly observed photospheric chemical
depletion. Using spectral energy distribution (SED) fitting and
period-luminosity-colour (PLC) relation methods, we determine the luminosity of
the targets ($2700-8300L_{\odot}$), which enables confirmation of their
evolutionary phase and estimation of initial masses (as a function of
metallicity) ($1-2.5M_{\odot}$). In conjunction with predictions from dedicated
ATON stellar evolutionary models, our results indicate a predominant intrinsic
enrichment of carbon and s-process elements in our binary post-AGB targets. We
qualitatively rule out extrinsic enrichment and inherited s-process enrichment
from the host galaxy as plausible explanations for the observed overabundances.
Our chemically peculiar subset of intrinsic carbon and s-process enriched
binary post-AGBs also hints at potential variation in the efficiency of
chemical depletion between stars with C-rich and O-rich circumbinary disc
chemistries. However, critical observational studies of circumbinary disc
chemistry are necessary to address gaps in our current understanding of
disc-binary interactions inducing chemical depletion in binary post-AGB
systems.
