Low-order Fourier parameters for RV curves of fundamental-mode and first-overtone Cepheids. For the fundamental mode, black points and gray points are variables of Quality (1+1a) and 2, respectively (see Table 1). Vertical dashed lined indicate a pulsation period of 4.68 and 10 days to guide the eye in the resonance vicinity of first-overtone and fundamental-mode Cepheids, respectively.