ATLAS o-band light curve obtained during the plateau, with the slow brightening trend removed (panel a). The second panel shows Fourier spectrum (black line) in the log-log scale, calculated from the o-band data taken during the plateau and transformed to normalised flux unit. The flickering noise nature (a_f ∼ f^−1/2) of these oscillations is indicated by the two parallel green dashed lines, while the amplitude errors are represented by blue dots. Third panel (c) shows respective WWZ spectrum. The major edge effects lie outside of the white dashed lines. The WWZ power is expressed in colours, as shown in the right label.