Hydrodynamic cycles in RR Lyrae stars

We present a detailed atmospheric dynamics study of Blazhko and nonBlazhko RR ab stars, using a high-resolution spectroscopic survey. We derived the dynamical parameters: dynamical accelerations, radius variations, rise time, shock amplitudes, full width at half maximum, and their variation during the Blazhko modulation, and we highlight the relationship between period changes and variation in dynamical parameters over multi- Blazhko cycles. We discuss the origin of the Blazhko effect, giving a new empirical side for further theoretical models.