Left panel: we show the normalised observed amplitude for f₁ (black curve) together with the errors on the data points (thin curves shifted downwards along the y axis for clarity). The theoretical monochromatic amplitude as calculated for a radial mode with P = 146.9 s and model parameters characteristic of EC 20338−1925 is overplotted (red curve). Right panel: similar to the left panel, but referring to the monochromatic phase. The theoretical monochromatic phase was computed using the same parameters as for the amplitude, and additionally a radial velocity shift of 11 km s^-1 was assumed.