On the Influence of ‘‘Red Leak’’ of Light Filters on the Brightness Estimates of Stars of Late Spectral Types Illustrated by the Observations of Rapid Variability of Symbiotic Stars

Results of the simulation of the dependence of the ‘‘red leak’’ magnitude of photometric filters on various factors during star observations are presented: the colour index \(V\)–\(R\), luminosity class, magnitude of interstellar reddening, air mass, and precipitable water vapour in the Earth’s atmosphere. The error arising from the neglect of ‘‘red leak’’ in the case of filters used on the 0.6-m telescope of the SAI CMO can be up to \(0.6^{m}\)–\(0.8^{m}\) for stars of late spectral types. Algorithms for the reduction of observational data for the \(U\) and \(B\) filters are presented. The results of observations of the rapid variability of two symbiotic stars CH Cyg and SU Lyn with cold components of very late spectral types are provided. For CH Cyg, rapid variability was detected on both observation dates. Taking into account the ‘‘red leak’’ effect, the amplitude of brightness changes in the \(B\) band was 0.10\({}^{m}\) on November 6, 2019 and 0.19\({}^{m}\) on December 15, 2022, with a characteristic variability time of about 20 min. For SU Lyn, rapid brightness variability in the \(B\) band on February 25, 2023 was not detected (with an accuracy of up to 0.003\({}^{m}\)).