5 Conclusion

We find that:

1.

The observed properties of the orbitally related variation in the LCs of symbiotic binaries - the large amplitude, shaping of minima and systematic changes in their positions - cannot be produced by the reflection effect.

2.

According to the basic ionization model of symbiotic binaries, the main source of the optical continuum is the nebula, which arises from ionization of the cool giant wind by the hot star radiation. We found that:

(a)

The observed EM is consistent with that produced by the ionization model.

(b)

Variation in the EM is fully responsible for the observed wave-like modulation of the LCs of symbiotic binaries with the orbital phase.

(c)

The variation in the optical continuum should always be followed by a similar variation in Balmer lines.

3.

To explain the orbitally-related variation, the nebula has to be partially optically thick and of a non-symmetrical shape to produce different contributions of its total emission into the line of sight at different orbital phases.

4.

The LC profile depends on the extent of the symbiotic nebula. Large nebulae ( ) produce a sinusoidal LC ( $a \sim 0.5$). Less extended nebulae ( ) have a flatter LC at its maximum, and/or create a secondary minimum, ( ), which mimics the ellipsoidal effect.

Acknowledgements

This research has been supported by the Slovak Academy of Science under a grant No. 5038/2000 and the Alexander von Humboldt foundation under project No. SLA/1039115. The author would like to thank his anonymous referee for constructive comments that allowed him to improve the original version of the present work.