  V473 Lyrae is the only Galactic Cepheid with confirmed periodic amplitude and
phase variations similar to the Blazhko effect observed in RR Lyrae stars. We
collected all available photometric data and some radial velocity measurements
to investigate the nature of the modulation. The comparison of the photometric
and radial velocity amplitudes confirmed that the star pulsates in the second
overtone. The extensive data set, spanning more than 40 years, allowed us to
detect a secondary modulation cycle with a period of approximately 5300 days or
14.5 years. The secondary variations can be detected in the period of the
primary modulation, as well.
  Phenomenologically, the light variations are analogous to the Blazhko effect.
To find a physical link, we calculated linear hydrodynamic models to search for
potential mode resonances that could drive the modulation and found two viable
half-integer (n:2) and three n:4 resonances between the second overtone and
other modes. If any of these resonances will be confirmed by non-linear models,
it may confirm the mode resonance model, a common mechanism that can drive
modulations both in RR Lyrae and Cepheid stars.
