Fourier amplitude ratios and phase parameters (R₂₁ to R₆₁ and ϕ₂₁ to ϕ₆₁) of Cepheid RV curves (a) and LCs (b) as a function of the period. Models for MU Cep and OGLE-LMC-CEP-227 are shown to illustrate the difference between solar and sub-solar metallicity around P = 4 day. The solar and sub-solar metallicity variables are plotted with yellow and red error bars (magenta and green, respectively) for the RV curves (LCs, respectively). Fourier parameters are derived with decomposition analysis presented in Sect. 3.