  The results of modeling the dependence of the red leak of photometric filters
on various factors (color index V-R, luminosity class, interstellar reddening,
airmass and PWV) during observations of stars are presented. The error arising
from not taking into account the red leak in the case of filters used on the
0.6-m telescope of the CMO SAI can amount to 0.6-0.8 mag for late stars.
Algorithms for reducing observational data are presented for filters U and B.
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 presented.
For CH Cyg, rapid variability was detected on both observation dates. Taking
into account the red leak effect, the amplitude in the B band was 0.10 mag on
November 6, 2019 and 0.19 mag on December 15, 2022, with a characteristic
variability time of about 20 minutes. For SU Lyn, no rapid brightness
variability was detected in the B band on February 2, 2023 (with an accuracy of
0.003 mag).
