Top panel: FT of the 516 data points obtained from the (O–C) DQ Vel photometric data. Middle upper panel: FT of the residuals after prewhitening the original data by the first significant frequency (f₁). Middle lower panel: FT of the residuals prewhitened by the two significant frequencies f₁ and f₂ listed in Table 7. The white solid line corresponds to σ_noise and the dotted line corresponds to 4σ_noise. Bottom panel: spectral window for the whole data set.