Upper panel: average I_line in the Ca II IRT line at 8500.35 Å against the average I_line in the Hα line of the investigated stars. The error bars correspond to the standard deviations of the line indices of individual spectra of the respective stars. Lower panel: zoom of the range of the inactive stars without error bars (blue box in the upper panel). The black pluses indicate the stars best fit by model #079, red hexagons indicate best fit by model #080, blue circles by #042, green diamonds by #047, and magenta squares by #029 (see Sect. 4.2). The model numbers and properties are given in Table C.1. To improve clarity, the errors are faded out in the lower panel.