Notices to investigation of symbiotic binaries IV. Optical light curves from the near-ultraviolet

The aim of this note is to reconstruct optical light curves (LC) from the near-UV fluxes of the ultraviolet spectra of symbiotic binaries during their quiescent phases. The method is based on the fact that the nebular component of radiation dominates the near-UV during quiescent phases and represents the main source of the light variability in the optical. We demonstrate this approach on example of two quiet symbiotic stars, SY Mus and RW Hya. Using their IUE spectra we determined U and B magnitudes in the standard Johnson system. Values derived from the near-ultraviolet are fainter then those measured photometrically by ΔU≈0.2 and ΔB≈0.4. This difference is due to emission lines.