What mimics the reflection effect in symbiotic binaries?

We discuss the origin of a periodic wave-like variation as a function of the 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 effect. For example, the observed amplitudes of the LCs are far larger than those 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.