First detection of a frequency multiplet in the line-profile variations of RR Lyrae: towards an understanding of the Blazhko effect

All present models for explaining the Blazhko effect are based on the presence of non-radial components. Up to now, however, these components have not been found explicitly in the observational studies, which were all based on photoelectric data. We provide the first detection of a frequency multiplet in the line-profile variations of RR Lyrae. Performing a period analysis on 669 high resolution line profiles obtained with the spectrograph ELODIE at OHP, we clearly detect a multiplet structure, with a separation equal to the Blazhko frequency, around the main frequency and its harmonics. Additional observations are needed to decide about the existence of a quintuplet. The presence of the harmonics of the main frequency in the line profiles indicates the importance of non-linear effects in the pulsation. The complete understanding of the origin of the Blazhko effect still needs further theoretical modelling and better new observations. Our detection of the frequency multiplet in the line-profile variations is a first essential step towards a decisive confrontation between the theoretical models and the observations.