First direct detection of an RR Lyrae star conclusively associated with an intermediate-age cluster

RR Lyrae stars have long been considered unequivocal tracers of old (>10 Gyr) and metal-poor ($\mathrm{[Fe/H]}<-0.5$) stellar populations. First, because these populations are where they are readily found and because, according to canonical stellar evolution models for isolated stars, these are the only populations where RR Lyrae should exist. Recent independent results, however, are challenging this view and pointing at the existence of intermediate-age RR Lyrae, only a few (2$-$5) Gyrs old. Our goal in this work is to provide direct evidence of the existence of intermediate-age RR Lyrae by searching for these stars in Milky Way open clusters, where the age association will be direct and robust. We searched a catalogue of over 3,000 open clusters with published kinematically associated member stars by crossmatching it against a compilation of the largest publicly available RR Lyrae surveys (Gaia, ASAS-SN, PanStarrs1, Zwicky Transient Facility and OGLE-IV). We identified one star as a bona fide RR Lyrae variable and robust member of the 2$-$4 Gyr old Trumpler 5 cluster, based on its parallax and proper motions and their agreement with confirmed cluster members. We derived an extremely low probability ($0.049\pm 0.013$%) that the star is a background field RR Lyrae and provide initial constraints on a possible binary companion based on its position in the colour-absolute magnitude diagram. Currently a source of debate, the Trumpler 5 RR Lyrae provides the most direct evidence to date of the existence of RR Lyrae stars at much younger ages than traditionally expected and adds to the mounting evidence supporting their existence.