RRab variable stars
RRc variable stars
Blazhko effect
A long term, generally irregular modulation of light curves of a large subclass of RR Lyrae stars. Most of the modulations occur on the time scale of some 60 periods, although the range extends from some tens to some hundreds of periods. Since its discovery over a hundred years ago, a number of explanations have been proposed for this effect, but its nature is still a matter of investigation. The explanations include: closely spaced pulsation modes, a modal 1:2 resonance, an oblique rotator model, a non-radial modal interaction, convective cycles, and nonlinear resonant mode coupling between the 9th overtone and the fundamental mode.
This is phenomenon observed in RR Lyr type stars and demonstrates as changes of the light curve on timescales longer than pulsation period. The effect was first found more than 100 years ago and its origin is still not well understood and subject of active research. - https://github.com/astrothesaurus/UAT/issues/286