Present Understanding of the Light Curves of Symbiotic Binaries

We explain a complex behaviour of the light curves (LC) of symbiotic binaries on the basis of their spectral energy distribution (SED) in the optical domain. During quiescent phases we observe a wave-like variation of the optical light as a function of orbital phase. This is produced by the orbitally-related variation of the emission measure (EM) of the symbiotic nebula. During active phases this type of variability disappears, the continuum level increases by 2–3 magnitudes in U and, in the case of a highly inclined orbit, narrow minima – eclipses – in the LC can occur. These phenomena are connected with the creation of a cool pseudo-photosphere around the hot star, whose contribution is significant in the optical, while the nebula practically disappears.