Top: phase-folded light curve of U Gru for sector 1. Middle: amplitude modulation of f₁₄ = 33.046 d⁻¹ as reconstructed according to the fitting method developed by Jayaraman et al. (2022). The blue considers all 14 significant frequencies split from f₁₄ listed in Table 3. The orange lines only consider those frequencies with f = f₁₄ ± Nf_orb, where −3 ≤ N ≤ 3, if they were found to be significant in the iterative pre-whitening procedure. Tracked mode amplitudes from Sect. 4 are shown as black crosses. Bottom: same as the middle panel but for the phase modulation. Grey hatching denotes phases of primary (forward hatching) and secondary (cross hatching) eclipse.