ℱ′_HK of selected giant stars in the range 0.76 ≤ B − V < 1.18, the lower envelope is labelled with a solid line. The dashed line shows the basal flux from Strassmeier et al. (1994) (multiplied by the factor 2.5) directly, the dash-dotted (Rutten 1984) and dotted lines (Rutten et al. 1991) show the basal fluxes one would obtain, if the photospheric contributions are subtracted from the minimal fluxes determined by Rutten in the cited papers.