Carnegie Observatories Astrophysics Series, Vol. 5: RR Lyrae Stars, Metal-Poor Stars, and the Galaxy ed. A. McWilliam (Pasadena: Carnegie Observatories) Peculiarities of Blazhko Stars: New Insights

With increasingly accurate data on RR Lyrae stars we find that the Blazhko effect may be a rule rather than an exception. However, we still do not know what is the cause of this mysterious amplitude and phase modulation. In my talk, I intended to give a glimpse of the properties of Blazhko stars, presenting recent findings concerning what makes Blazhko stars different from their non-modulated counterparts. Recent observa-tions of Blazhko stars obtained from space give some important clues that deserve further exploration. 1. A century-old mystery In 1907, Sergei Nicolaevich Blazhko reported on the periodic variations in the timing of maximum light for the star RW Dra (Blazhko 1907). Soon after that, it was realized that also other, similar stars, showed modulations of their light curve shape over time scales of weeks or even months. In 1916 Harlow Shapley reported ”on the changes in the spectrum, period and light curve of the Cepheid variable RR Lyrae”, the star that later became the eponym of a new, distinct, class of variables. Szeidl (1988) reported an occurrence rate of 20-30 % for Galactic RRab stars and only a few percent for RRc stars. In the LMC this occurrence rate appears to be somewhat lower (12 % RRab and 4 % RRc, see Alcock et al. 2003, 2000 respectively), though high-precison surveys may slightly change these numbers. The most recent surveys, both from the ground (Jurcsik et al. 2010) and from space (Szabo ́ et al. 2010a, Benko ̋ et al. 2010), seem to indicate that close to 50 % of the fundamental mode RR Lyrae pulsators show the Blazhko effect. Typical light curves of a non-Blazhko and a Blazhko variable obtained Electronic address: (2