  We propose a simple dynamical model that may account for the observed
spectral and temporal properties of GRS 1915+105 and XTE J1550-5634. The model
is based on the assumption that a fraction of the radiation emitted by a hot
spot lying on the accreting disk is dynamically Comptonized by the relativistic
jet that typically accompanies the microquasar phenomenon. We show that
scattering by the jet produces a detectable modulation of the observed flux. In
particular, we found that the phase lag between hard and soft photons depends
on the radial position of the hot spot and, if the angle between the jet and
the line of sight is sufficiently large, the lags of the fundamental and its
harmonics may be either positive or negative.
