  Further evidence is presented supporting the alternative interpretation of
superhumps as being due to irradiation modulated periodically variable mass
transfer rate. NZ Boo, HT Cas and PU UMa are added to the sample of high
inclination dwarf novae showing -- during their superoutbursts -- modulation of
the observed brightness of the disk with beat period. Simple model calculations
confirm earlier hypothesis that this modulation is due to a non-axisymmetric
structure of the outer parts of the disk, involving the azimuthal dependence of
their geometrical thickness, rotating with the beat period.
  The modulation amplitude $A_{mod}$ is found to decrease during
superoutbursts. In particular, it is found that during two superoutbursts of OY
Car the rate of decline of the superhump amplitude $dA_{SH}/dt$ was correlated
with the rate of decline $dA_{mod}/dt$. This leads to a simple explanation for
the decreasing amplitudes of superhumps: it is a consequence of the decreasing
modulation amplitude.
