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Barium stars are s-process enriched giants. They owe their chemicalpeculiarities to a past mass transfer phase. During which they were polluted bytheir binary companion, which at the time was an AGB star, but is now anextinct white dwarf. Barium stars are thus ideal targets for understanding andconstraining the s-process in low and intermediate-mass AGB stars. We derivethe abundances of a large number of heavy elements in order to shed light onthe conditions of operation of the neutron source responsible for theproduction of s-elements in the former companions of the barium stars. Adoptinga recently used methodology(Neyskens et al. 2015), we analyse a sample of 18highly enriched barium stars observed with the high-resolution HERMESspectrograph mounted on the MERCATOR telescope (La Palma). We determine thestellar parameters and abundances using MARCS model atmospheres. In particular,we derive the Nb/Zr ratio which was previously shown to be a sensitivethermometer for the s-process nucleosynthesis. Indeed, in barium stars,$^{93}Zr$ has fully decayed into mono-isotopic $^{93}Nb$ , so Nb/Zr is ameasure of the temperature-sensitive $^{93}Zr/Zr$ isotopic ratio. HD 28159,previously classified as K5III and initially selected to serve as a referencecool K star for our abundance analysis, turns out to be enriched in s-processelements, and as such is a new barium star. Four stars, characterised by highnitrogen abundances, also tend to have high [Nb/Zr] and [hs/ls] ratios. Thederived Zr and Nb abundances provide more accurate constraints on the s-processneutron source, identified to be $^{13}C(alpha,n)^{16}O$ for barium stars. Thecomparison with stellar evolution and nucleosynthesis models shows that theinvestigated barium stars were polluted by a low-mass (2-3 Solar mass) AGBstar. HD 100503 is potentially identified as the highest metallicity CEMP-rsstar yet discovered.

When binaries keep track of recent nucleosynthesis: The Zr - Nb pair in extrinsic stars as an s-process diagnostic