  RR Lyrae pulsating stars have been extensively used as tracers of old stellar
populations for the purpose of determining the ages of galaxies, and as tools
to measure distances to nearby galaxies. There was accordingly considerable
interest when the RR Lyr star OGLE-BLG-RRLYR-02792 was found to be a member in
an eclipsing binary system4, as the mass of the pulsator (hitherto constrained
only by models) could be unambiguously determined. Here we report that
RRLYR-02792 has a mass of 0.26 M_sun and therefore cannot be a classical RR
Lyrae star. Through models we find that its properties are best explained by
the evolution of a close binary system that started with 1.4 M_sun and 0.8
M_sun stars orbiting each other with an initial period of 2.9 days. Mass
exchange over 5.4 Gyr produced the observed system, which is now in a very
short-lived phase where the physical properties of the pulsator happen to place
it in the same instability strip of the H-R diagram occupied by RR Lyrae stars.
We estimate that samples of RR Lyr stars may contain a 0.2 percent
contamination with systems similar to this one, implying that distances
measured with RR Lyrae stars should not be significantly affected by these
binary interlopers.
