Fourier parameters of the final sample of the 545 fundamental-mode Cepheids in the V band with spectroscopic metallicities. The different colors refer to metallicity above and below the median of the sample, as indicated in panel (a). The peculiar appearance of ϕ₄₁ with a break around seven days was noted by Simon & Moffett (1985), Kovacs et al. (1990). The latter proposed an unknown atmospherical effect since these authors did not observe this break in the radial velocity curves. (a) ϕ₂₁, (b) R₂₁, (c) ϕ₃₁, (d) R₃₁, (e) ϕ₄₁, (f) R₄₁.