O − C diagram (top) and amplitude changes (bottom) of the n = 25 TEO observed in the combined ground-based data from the MACHO, OGLE-III, OGLE-IV, and ASAS-SN surveys and the TESS space mission. Vertical stripes mark the time span of the light curves from different ground-based surveys. The solid red curve in the upper panel shows the values of Δf defined by Eq. (3), expressed in terms of the orbital frequency (in per cent). The units of Δf/f_orb are shown as a right-hand ordinate. The grey shaded area around the curve represents the ±1σ confidence interval derived from the Monte Carlo simulations. More details are given in the main text.