  Context. Abundance analysis of post-AGB objects as probes of AGB
nucleosynthesis. Aims. A detailed photospheric abundance study is performed on
the carbon-rich post-AGB candidate MACHO47.2496.8 in the LMC. Methods.
High-resolution, high signal-to-noise ESO VLT-UVES spectra of MACHO47.2496.8
are analysed by performing detailed spectrum synthesis modelling using
state-of-the-art carbon-rich MARCS atmosphere models. Results. The spectrum of
MACHO47.2496.8 is not only dominated by bands of carbon bearing molecules, but
also by lines of atomic transitions of s-process elements. The metallicity of
[Fe/H]=-1.4 is surprisingly low for a field LMC star. The C/O ratio, however
difficult to quantify, is greater than 2, and the s-process enrichment is
large: the light s-process elements are enhanced by 1.2 dex compared to iron
([ls/Fe]=+1.2), while for the heavy s-process elements an even stronger
enrichment is measured: [hs/Fe]=+2.1. The lead abundance is comparable to the
[hs/Fe]. With its low intrinsic metallicity and its luminosity at the low end
of the carbon star luminosity function, the star represents likely the final
stage of a low initial mass star. Conclusions. The LMC RV Tauri star
MACHO47.2496.8 is highly carbon and s-process enriched, and is most probable a
genuine post-C(N-type) AGB star. This is the first detailed abundance analysis
of an extragalactic post-AGB star to date.
