Left panel: we show the observed amplitude as a function of wavelength for the dominant pulsation mode f₁. For reference we plot the averaged spectrum of EC 20338−1925 on an arbitrary y-axis scale and also indicate the central wavelengths of the Balmer and prominent He lines (dotted vertical lines). The thicker yellow bars mark the wavelength bins for which the Fourier Transforms plotted in Fig. 6 were computed. Right panel: the same as the left panel, but for the observed phase.