1998A&A...338..599S

We present long-term photometric observations of a group of classical symbiotic stars: BF Cyg, CI Cyg, V1329 Cyg, AG Peg, Z And, AG Dra and YY Her. Analysis of the minima positions in the historical light curves revealed a systematic variation in the O-C residuals which indicates apparent changes of the orbital period in these systems. It was found that (i) this variation is connected with changes in the energy distribution of the hot star spectrum, and (ii) the phase shift in the period correlates with the change in the star's brightness. The effects are best demonstrated by the eclipsing systems of BF Cyg and CI Cyg. During quiescent phases, when the nebular radiation dominates the optical spectrum, the minima occur prior to the time of the inferior conjunction of the giant star. However, during active phases, when the component of recombination radiation decreases considerably and a cool shell around the hot star developes, the minima - eclipses - coincide with spectroscopic conjunction of the stars. The observed variation is interpreted as a result of systematic changes in both geometry and location of the main source of the optical continuum caused by a variation in the luminosity of photons capable of ionizing hydrogen during transition periods between different levels of activity of symbiotic binary.