GP periodograms for the long-period searches (3–29 h) from the TESS Sector 9, 36, 63, and 90 photometry. Left column: 1D periodograms for a single-period GP model. The plotted signal amplitude ΔBIC is defined as the difference between the BIC of the single-period GP model and that of the aperiodic GP model, i.e. ΔBIC = BIC₁(p₁) − BIC₀. Right column: conditional 1D periodograms for a two-period GP model where p₁ is fixed to 25.01 h. Here, ΔBIC = BIC₂(p₁ = 25.01 h, p₂) − BIC₁(p₁ = 25.01 h). The known 25.01 h period and its harmonics are marked with solid and dashed blue lines, respectively, while the 11.4 h period and its harmonics are marked with the solid and dotted green lines. The horizontal slashed line shows the ΔBIC level of −10, which corresponds to very strong evidence in favour of the periodic signal (Kass & Raftery 1995).