Periodogram of the He ii λ 4686 emission line as observed in August 2013. In the mean periodogram over the wavelength interval 4678−4692 Å, as computed from the time series of HEROS spectra (top panel), the highest peak is found at the frequency ν₁ = 1.315 day^-1. The residual periodogram, after prewhitening the data for the signal corresponding to the ν₁ frequency, is shown in the middle panel. The bottom panel provides the power spectral window of our time series.