GLS periodograms of individual wavelength contrast light curves. In each of the panels, the light pink, dark pink, and black highlighted lines are the first, second, and third ¹²CO band heads, respectively. The horizontal grey lines indicate the 1%, 5%, and 10% false alarm probabilities. The vertical black lines indicate the expected rotation period at 8.7 hrs and its first integer harmonic at 4.35 hrs. Top: Uncorrected periodogram, with all wavelengths significantly affected by systematic effects caused by the fibre coupling. Middle: Periodogram after the polynomial correction. Bottom: Gaussian smoothing added to the periodogram to highlight the strong periodic signals.