  In this research, five eclipsing binary stars were studied: AB Cas, AF Gem,
AR Boo, BF Vir and CL Aur. The large sets of moments of minima were used: from
the international BRNO database and amateur observations from the database
AAVSO. Firstly, moments of minima for AAVSO observations were obtained (totally
- 222 minima). The software MAVKA was used. It was kindly provided by Kateryna
D. Andrych and Ivan L. Andronov (2019OEJV..197...65A) and approximation with
various methods in order to find the best fit. Then all obtained moments of
minima were combined and O-C diagrams were plotted. For all stars these
diagrams represented sinusoidal-like oscillations with superposition of
parabolic trend. One of the possible reasons for such oscillations could be
presence of well-known light-time effect (LTE) caused by third component with
elliptical orbit. Parabolic trend was explained as mass transfer between
components of binary system. For all these stars we computed possible mass of
the third component, orbital elements, mass transfer rate and errors for all
computed values.
