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Published online by Cambridge University Press: 12 April 2016
The subject “Multiperiodic RR Lyrae Stars” encompasses two entirely different types of phenomena: 1) long-term modulation of amplitude known as the Blazhko-effect and 2) simultaneous excitation of several radial modes. In the first category the observational material is now quite extensive, but very little theoretical analysis has been undertaken. In the second case the results of pulsation theory are applicable, but only one such object has been found. Much progress in our understanding of RR Lyrae stars has been made in recent years, however, and many results do have a bearing on the present problem. Several excellent reviews of current pulsation theory are available (see Iben 1971c, or Cox 1974a,b). I will therefore restrict my comments to those results bearing directly on choice of mode for RR Lyrae stars.