Distribution of observed brightness in K_p band (top panel) and first-overtone period (bottom panel) for stars classified as RRc (blue solid line). With a red dashed line, we plotted the distributions for RRc stars in which we detected additional low-amplitude signals. We also provide the incidence rate of RRc stars with the additional signals for each bin of both distributions. The error was calculated assuming Poisson distribution (see e.g. Alcock et al. 2003).