Sliding periodograms (slidograms) in two frequency ranges calculated for the TESS-21 data set, shown as gray-scale images. The slidograms in a low-frequency range of 0.1–1.1 d⁻¹ are calculated with a window of 10 days, while for frequencies close to the orbital one, we used the window of 2 days. The right panel shows the light curve, also smoothed with the window of 2 days. Dashed vertical lines in the slidograms denote the important frequencies discussed in the paper (see text for explanation).