Blue spectrum of Cyg OB2 #5 as a function of orbital phase as observed with the Aurélie spectrograph during our June 2014 OHP campaign. For clarity the consecutive spectra are shifted vertically by 0.3 continuum units. Right panel: zoom on the He II λ 4542 line. The blue and red tick marks above the spectra indicate the position of the primary and secondary absorptions, respectively, for those spectra where the lines were resolved.