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In this paper it is shown that the Blazhko effect may result from anear-resonant type of multi-mode pulsation, where two (or sometimes more)periodic oscillations with slightly different frequencies gradually slip inphase, producing a beat frequency type of modulation. Typically one of theseoscillations is strongly non-sinusoidal. Two oscillations are sufficient forthe standard Blazhko effect; additional oscillations are needed to explainmulti-frequency modulation. Previous work on this hypothesis by Arthur N. Coxand others is extended in this paper by developing a simple (non-hydro) modelthat can accurately reproduce several important features found in Kepler datafor RR Lyr, including the pulsation waveform, the upper and lower Blazhkoenvelope functions and the motion, disappearance and reappearance of the bumpfeature. The non-sinusoidal oscillation is probably generated by thefundamental mode and the other oscillations are probably generated by nonradialmodes. This model provides an explanation for the strong asymmetry observed inthe side peak spectra of most RR Lyrae stars. The motion and disappearance ofthe bump feature are shown to be an illusion, just an artifact of combining theoscillations. V445 Lyr is presented as an example with dual modulation. Themysterious double-maxima waveform observed for this star is explained,providing additional strong evidence that this beating-modes hypothesis iscorrect. Problems with other recent explanations of the Blazhko effect arediscussed in some detail.

Is the Blazhko effect the beating of a near-resonant double-mode pulsation?