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 MACHO 47.2496.8 in the LMC .

Methods : High-resolution , high signal-to-noise ESO VLT-UVES spectra of MACHO 47.2496.8 are analysed by performing detailed spectrum synthesis modelling using state-of-the-art carbon-rich MARCS atmosphere models .

Results : The spectrum of MACHO 47.2496.8 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 MACHO 47.2496.8 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 .