No country for old stars

Context. RR Lyrae (RRL) stars are widely considered tracers of ancient (> 10 Gyr) metal-poor stellar populations. However, recent kinematic and photometric studies suggest the existence of a metal-rich RRL subpopulation associated with the thin disk and intermediate ages (∼2–5 Gyr), therefore challenging canonical evolutionary models. Aims. We aim to provide the first spectroscopic confirmation of a member of this elusive population. Specifically, we target a metal-rich RRL candidate recently identified photometrically as a member of the intermediate-age open cluster Trumpler 5 (∼2.5 Gyr). Methods. We obtained high-resolution spectroscopy using PEPSI at the LBT and GHOST at Gemini South telescope. We measured radial velocities (RVs) from multiple epochs to constrain cluster membership and derived detailed chemical abundances (Mg, Ca, Sc, Ti, Mn, Fe, Cu, Zn, Y, and Ba) to compare the RRL’s composition with that of red clump stars in the cluster. Results. The RRL’s systemic velocity ($ V_\gamma = 50.57^{+0.78}_{-0.36} $ km s−1) is in excellent agreement with the cluster mean (V = 50.76 ± 0.49 km s−1). Combining RVs, proper motions, and parallax, the probability of the star being a background interloper is negligible (∼0.002%, better than 4σ). We derived a metallicity of [Fe/H] = −0.40 ± 0.05, which matches the cluster value. While most abundance ratios (Mg, Ti, Mn, Cu, and Zn) align with cluster members, the RRL exhibits significant depletion in Ca, Sc, Y, and Ba. Notably, [Sc/Fe] is underabundant by ∼0.6 dex relative to the cluster stars, following trends seen in field metal-rich RRLs. Conclusions. We provide strong constraints on the membership status between an RRL variable and an intermediate-age open cluster. Cluster membership enables accurate measurement of RRL age and chemical anomalies relative to its host, particularly in Sc and neutron-capture elements. These anomalies further reinforce a nonstandard formation channel for this RRL, possibly indicating binary interactions and mass transfer.