We discuss the origin of a periodic wave-like variation as a function of th
e orbital phase observed in the light curves (LC) of symbiotic stars. It is
shown that this type of variability cannot be ascribed to a reflection eff
ect. For example, the observed amplitudes of the LCs are far larger than th
ose calculated within a model of the reflection effect. Here, the nature of
the orbitally related changes in the optical continuum is outlined within
an ionization model of symbiotic binaries.