  The results of three-dimensional numerical simulations of mass transfer in
binaries with spiral shocks are presented. It is shown that the mass transfer
rate variation disturbs the equilibrium state of accretion disk and results in
formation of the blob. This dense formation moves on the disk with variable
velocity. The results of our simulations show that the blob lives long enough
and retains its main characteristics for the time of order of tens orbital
periods. Analysis of the results shows that the action of dissipation is
negligible on these timescales and the blob smearing out due to differential
rotation of the disk is stopped by its interaction with spiral shocks.
  Light curves of some CVs show periodic or quasi-periodic photometric
modulations with typical period 0.1-0.2P_orb. We suggest to consider the
formation of the blob in binaries with spiral shocks as a possible reason for
these modulations. The efficiency of this mechanism is confirmed by good
qualitative (and, in part, quantitative) agreement between the results of our
simulations and observations of binaries IP Peg and EX Dra.
