  The phase lag structure between the hard and soft X-ray photons observed in
GRS 1915+105 and XTE J1550+564 has been said to be ``complex'' because the
phase of the Quasi-Periodic Oscillation fundamental Fourier mode changes with
time and because the even and odd harmonics signs behave differentely.
  From simultaneous X-ray and radio observations this seems to be related to
the presence of a jet (level of radio emission). We propose a simple idea where
a partial absorption of the signal can shift the phases of the Fourier modes
and account for the phase lag reversal. We also briefly discuss a possible
physical mechanism that could lead to such an absorption of the quasi-periodic
oscillation modulation.
