Pulsation amplitude as a function of the mean brightness for the Kepler data of DF Cyg. Top panel: instantaneous flux amplitudes versus mean fluxes in linear scaling. The dotted red line shows a linear fit to the data, which was used to estimate the and parameters. The formal error bars are smaller than the symbol sizes. Bottom panel: the relative pulsation amplitude versus the relative mean flux in equidistant bins. Here the vertical error bars show the standard deviations of the mean, while the horizontal error bars indicate the width of the bins (note the missing bin at 0.7 due to lack of points). The diagonal black lines show the line of equality in both panels. See text for details.