Oscillatory behavior of the activity index time series and their corresponding power spectra, obtained for KIC 12365719 with P = 0.854 days and T_eff = 3556 K (panels A and B), for KIC 8144578 with P = 0.593 days and T_eff = 6641 K (panels C and D), and for KIC 7671028 with P = 15.623 days and T_eff = 6675 K (panels E and F). The abscissa units ΔJD in panels A, C, and E are the Julian days counted from JD 2454833. The peak of maximum amplitude is indicated by an arrow and labeled P_peak.