The Blazhko effect, which is sometimes called long-period modulation, is a variation in period and amplitude in RR Lyrae type variable stars. It was first observed by Sergey Blazhko in 1907 in the star RW Dra. The recent Kepler (spacecraft) observations caught the community by surprise.  In addition to the modulations the observations showed cycle to cycle alternations in the light curves.
The physics behind the Blazhko effect is currently still a matter of debate, with there being three primary hypotheses. In the first, referred to as the resonance model, the cause of the modulation is a non-linear resonance among either the fundamental or the first overtone pulsation mode of the star and a higher mode.   The second, known as the magnetic model, assumes the variation to be caused by the magnetic field being inclined to the rotational axis, deforming the main radial mode.  The third model assumes that cycles in the convection cause the alternations and the modulations. 
- Horace A. Smith (2004). RR Lyrae Stars. Cambridge University Press. p. 103. ISBN 0-521-54817-9.
- R. Szabo et al. , 2010, Monthly Notices of the Royal Astronomical Society, Vol. 409, p. 1244
- Z. Kollath, L. Molnar & R. Szabo, 2011, Monthly Notices of the Royal Astronomical Society, Vol 414 p. 1111; (also http://xxx.lanl.gov/abs/1102.0157)
- J. R. Buchler & Z. Kollath, 2011, Astrophysical Journal, Vol 731, p. 24 (also http://xxx.lanl.gov/abs/1101.1502)
- Katrien Kolenberg (2008). "Explanations for the Blazhko effect in RR Lyrae stars". The Blazhko Project. Retrieved 2008-12-17.
- Stothers, R.B. 2010, Publications of the Astronomical Society of he Pacific Vol. 122, p. 536
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