Left panel: the multiband LS periodograms based on the ZTF light curves, at which the highest peaks corresponding to P _n (upper panel) and P _c (lower panel). The vertical dashed line indicates the literature period of P _l = 156 days. Middle panel: folded r-band light curves with our determined period P _n (upper panel) and literature period P _l (lower panel). The filled and open circles are data points with and without color-terms correction, respectively. Right panel: ZTF light curves and color curve for NGC 6638 V15, where the symbols are same as the right panels of Figure 2.