Linear fit (purple line) of the SBCR (F_V, V − K) obtained with V and K magnitudes. The seven different Cepheids used to calibrate this SBCR are shown by different symbols. The dotted blue line corresponds to the relation found by Salsi et al. (2021) for giant stars. The dotted red line corresponds to the SBCR found by Kervella et al. (2004a), for Cepheids. The grey area indicates the RMS of the fit. The residual of the fit and the RMS are plotted in the lower panel.