Existing radial velocity data of 57 type I Galactic Cepheids are analyzed to study the systematic variation of their Fourier decomposition with the period. All important features (including the bump progression) of the radial velocity variation are described by low-order (third-order to fifth-order) Fourier decompositions. The data are in fair agreement with the recent hydrodynamic results, which implies that the 2:1 resonance between the fundamental and second overtone modes is the most important factor in the shaping of the radial velocity curves. The highest quality data of this sample suggest a very tight progression of the Fourier coefficients, which indicates strict constraints on the physical parameters or on the evolutionary history of Cepheids.
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Kovács et al. (1990) studied this question.