UBVRI photometry of the eclipsing symbiotic system CI Cygni in 1996-1999

The UBVRI photometry of the eclipsing symbiotic star CI Cyg in 1996-1999 is presented. The system continued to be in quiescence during this period. Th e shape of the minima in its light curves attributable to eclipses of the c ompact star with an accretion disk by the red giant is the same as that in 1988-1995, An analysis of our observations and those of other researchers, which span a total of 27 years, has revealed a cyclic variability of the ou t-of-eclipse photometric properties of CI Cyg on a time scale of about 10.7 +/- 0.6 years with the dearest manifestation in the U-B color. The fact th at the system's out-of-eclipse light variations in U, on the one hand, and in BVRI, on the other, occur in antiphase suggests that the titanium red gi ant plays a significant role in this cyclic process. However, with its cont ribution to the total U flux being no larger than 10%, the observed light f luctuations of CI Cyg in this band must be caused not only by variability o f the giant but also by light variations of the hot component. The presence of a 10.7-year cycle in the system's active and quiescent states suggests that some precession phenomenon is responsible for it. Precession of the ac cretion disk, which would cause both the observed brightness of the primary component and the effect of its radiation on the titanium red giant to var y, can serve as an example of such a phenomenon. (C) 2000 MAIK "Nauka/Inter periodica".