Almost a century after its discovery, the phenomenon of amplitude and/or phase modulation, observed in a large percentage of the RR Lyrae stars, still lacks a widely accepted theoretical understanding. Recent attempts to theoretically explain the effect focus on two alternatives, the magnetic models and the resonances models, both involving the presence of nonradial pulsation components.

We present the ‘Blazhko project’, a large international collaboration focused on understanding the Blazhko effect. The aim of the ‘Blazhko project’ is to combine spectroscopic and photometric data from a sample of well-selected Blazhko and nonBlazhko stars, to reveal decisive information on the physical mechanism responsible for the modulation.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html