Regression slope of the dependence of photometric brightness variation on HK emission variation, plotted vs. mean chromospheric activity calculated for α = 0 (original A_S coverages given by Eq. (1), solid curve), α = 0.5 (increased A_S coverages, dotted curve), α = − 0.5 (decreased A_S coverages, dashed curve). The calculations are performed for homogeneous distribution of active regions.