  We present non-linear hydrodynamic pulsation models for OGLE-BLG-RRLYR-02792
- a 0.26M_sun pulsator, component of the eclipsing binary system, analysed
recently by Pietrzynski et al. The star's light and radial velocity curves
mimic that of classical RR Lyrae stars, except for the bump in the middle of
the ascending branch of the radial velocity curve. We show that the bump is
caused by the 2:1 resonance between the fundamental mode and the second
overtone - the same mechanism that causes the Hertzsprung bump progression in
classical Cepheids. The models allow to constrain the parameters of the star,
in particular to estimate its absolute luminosity (approx 33L_sun) and
effective temperature (approx 6970K, close to the blue edge of the instability
strip).
  We conduct a model survey for the new class of low mass pulsators similar to
OGLE-BLG-RRLYR-02792 - products of evolution in the binary systems. We compute
a grid of models with masses corresponding to half (and less) of the typical
mass of RR Lyrae variable, 0.20M_sun<=M<=0.30M_sun, and discuss the properties
of the resulting light and radial velocity curves. Resonant bump progression is
clear and may be used to distinguish such stars from classical RR Lyrae stars.
We present the Fourier decomposition parameters for the modelled light and
radial velocity curves. The expected values of the phi_31 Fourier phase for the
light curves differ significantly from that observed in RR Lyrae stars, which
is another discriminant of the new class.
