Mirrored asymmetric line profiles and their Fourier transforms. Example from CPD−58°2611 in the He I 5876 line. Top: Original line profile with an asymmetric blue wing (black dashed), as well as the three types of symmetric line profiles: mirroring the red (v > 0 km/s) and blue (v < 0 km/s) halves of the line profile, and in green the averaged mirrored line profile. Middle: FT amplitude of the original line profile (black dashed), and the mirrored profiles (red, blue, green). Bottom: FT of the original asymmetric profile resembles a smooth chain of data points in the complex plane with non-zero imaginary part. The data point of the first minimum of the FT amplitude is marked in red.