Internal accuracy of reconstructed RV curves for calibrating Cepheids and for metal-poor Cepheids (yellow circles and red crosses, respectively). (a) Reconstruction residuals, σ_rms(V_r), for individual stars, as defined by Eq. (8). The dashed orange line indicates the median of the residuals for the calibrating sample. (b) Comparison of absolute integrated RV curves, as defined in Eq. (9). In the lower panel, we plot the differences between the two values. The dashed red line indicates the median of the differences, with the shaded green area marking their RMS. (c) Relative peak-to-peak amplitude difference between the reconstructed RV curves and true RV curves (Fourier fits), as defined in Eq. (10). The dashed blue line indicates the median value for the calibrating sample. (d) Accuracy of the RV curve reconstruction for Cepheids of the calibrating sample, versus the Fourier order (see also Table 2). The different lines represent the four previously discussed measures of accuracy, plotted with the same colours as in Figs. 9a–c (see Sect. 5.3).