Top panels: overplots of He ii λ4686 spectra, showing dramatic changes on short and long timescales. The spectra have been rebinned such that the S/N equals 810 in all four panels. Vertical dashed lines are drawn at ± vsini. Lower panels: temporal variance spectra expressed as the ratio of the observed to the expected standard deviation, σ_obs/σ_exp (similar to TVS^1/2). The lower long-dashed horizontal line is drawn at unity. The upper horizontal short-dashed line indicates the 1% significance level. From left to right: a) acquired at Calar Alto and Kitt Peak in 1989 with shortest time resolution of about 15 min. The corresponding periodogram is in the left-hand panel of Fig. 4; b) acquired at BOAO in 2007, spanning 27 days; c) acquired at the Mercator telescope in 2012 with its corresponding periodogram shown in the middle panel of Fig. 4; d) acquired at the Mercator telescope in 2013. The corresponding periodogram is in the right-hand panel of Fig. 4.