Left panel: Fourier transform of the line profile for different wavelength windows. The black line corresponds to the first minimum of the Fourier transform for the wavelength window shown in the right-hand panel. The coloured vertical dashed lines represent the first minimum of the Fourier transform obtained by adjusting the base window by ±0.1 Å the line profile. Right panel: Line profile for the He I+ IIλ4026 line in the stacked spectrum, with the selected wavelength window highlighted in orange.