Rare find: Discovery and chemo-dynamical properties of two s-process-enhanced RR Lyrae stars

Aims. We report the serendipitous discovery of two RR Lyrae stars that exhibit significant s-process element enrichment, a rare class previously represented solely by TY Gruis. Our goal is to characterise these objects chemically and dynamically, and explore their origins and evolutionary histories. Methods. Using high-resolution spectroscopy from HERMES@AAT and UVES@VLT, we derived detailed chemical abundances of key s-process elements (Y, Ba, La, Ce, Nd, and Eu) and carbon, along with α elements (Ca, Mg, and Ti). We also employed Gaia Data Release 3 astrometric data to analyse their kinematics, orbital properties, and classify their Galactic population membership. We compared observational results with theoretical asymptotic giant branch (AGB) nucleosynthesis models to interpret their enrichment patterns. Results. Both stars exhibit clear signatures of s-process enrichment, with significant overabundances in second-peak elements such as Ba and La compared to first-peak Y and Zr. Comparison with AGB nucleosynthesis models suggests their progenitors experienced pollution of s-process-rich material, consistent with early binary interactions. However, notable discrepancies in dilution factors highlight the need for more refined low-metallicity AGB models. We also explore and discuss alternative scenarios, including sub-luminous post-AGB-like evolution or double episodes of mass transfer. In the latter case, the star initially undergoes a mass transfer when it is on the main sequence, accreting material from a former AGB companion. Subsequently, as the star evolves along the red giant branch, it may again transfer mass to its companion before becoming an RR Lyrae star. Conclusions. Our findings confirm the existence of s-process-enhanced RR Lyrae stars and demonstrate the importance of combining chemical and dynamical diagnostics to unveil their complex evolutionary pathways. Future detailed binary evolution modelling and long-term orbital monitoring are essential to resolve their formation scenarios and assess the role of binarity in the evolution of pulsating variables.