Standard deviation of S_ph as a function of the average S_ph for main-sequence stars of different spectral types and subgiants, color-coded by the number of stars. The blue star marks the solar values from the 24 yr VIRGO g+r, while the blue circles indicate the values obtained from 4 yr segments. The turquoise star highlights Doris (KIC 8006161), a solar analog with a confirmed 8 yr activity cycle. The turquoise line indicates the range between minimum and maximum individual S_ph values. The squares mark three activity-cycle candidates: KIC 5184732, KIC 7970740, and KIC 10644253. The number of Kepler targets of each type is indicated in each panel.