Fourier frequency spectra for the combined blue- and red-filter B-C data of β Cen at four steps of the pre-whitening procedure. The detected terms are marked with inverted triangles and labeled from f₁ to f₁₉. The open triangles for f₁ and f₁₉ indicate combination frequencies. f_N ≈ 7.17 d^-1 stands for the Nyquist frequency related to the orbital sampling and is marked with a dashed line. The dotted vertical lines indicate frequencies of terms that were subtracted. Note the different ordinate scales in different panels.