Time–frequency diagram of Sector 41 TESS data of HD 192639. The top panel displays the TESS light curve. The bottom left-hand panel illustrates the evolution of the Fourier periodogram, with the date corresponding to the middle of the 10 d sliding window. The violet, dark blue, light blue, and cyan colours, respectively, represent areas with amplitudes ≥6, ≥4, ≥3, and ≥2mmag. The bottom right panel provides the Fourier periodogram evaluated over the full light curve of Sector 41. The various coloured lines correspond to the colour-scale used in the bottom left panel. The tick marks in the top panel indicate the times of our TIGRE observations. Those in colours correspond to the trough event.