  RR Lyrae are a well-known class of pulsating horizontal branch stars widely
used as tracers of old, metal-poor stellar populations. However, mounting
observational evidence shows that a significant fraction of these stars may be
young and metal-rich. Here, through detailed binary stellar evolution
modelling, we show that all such metal-rich RR Lyrae can be naturally produced
through binary interactions. Binary companions of these RR Lyrae stars formed
through binary interactions partly strip their progenitor's envelopes during a
preceding red giant phase. As a result, stripped horizontal branch stars become
bluer than their isolated stellar evolution counterparts and thus end up in the
instability strip. In contrast, in the single evolution scenario, the stars can
attain such colours only at large age and low metallicity. While binary-made RR
Lyrae can possess any ages and metallicities, their Galactic population is
relatively young (1 to 9 Gyr) and dominated by the Thin Disc and the Bulge. We
show that Galactic RR Lyrae from binary evolution are produced at rates
compatible with the observed metal-rich population and have consistent G-band
magnitudes, Galactic kinematics and pulsation properties. Furthermore, these
systems dominate the RR Lyrae population in the Solar Neighbourhood. We predict
that all metal-rich RR Lyrae have an A, F, G or K-type companion with a long
orbital period (P > 1000 d). Observationally characterising the orbital periods
and masses of such stellar companions will provide valuable new constraints on
mass and angular momentum-loss efficiency for Sun-like accretors and the nature
of RR Lyrae populations.
