  In our research, we studied 9 eclipsing binary stars: AR Lac, U CrB, S Equ,
SU Boo, VV UMa, WW Gem, YY Eri, V0404 Lyr, HP Aur. We collected large sets of
moments of minima from BRNO and observational data from AAVSO databases. Then,
we obtained moments of minima for AAVSO observations (totally - 397 minima)
using method of approximation with symmetric polynomial, realized in software
MAVKA. This software was provided by Kateryna D. Andrych and Ivan L. Andronov.
After we combined obtained moments of minima with data taken from BRNO and
plotted O-C diagrams. For all stars these diagrams represented sinusoidal-like
oscillations with superposition of linear (for SU Boo, VV UMa, WW Gem, V0404
Lyr and HP Aur) or parabolic trend (for AR Lac, U CrB, S Equ and YY Eri). The
oscillations could be described as presence of the third component, which does
not take part in eclipses, but causes the well-known light-time effect (LTE).
That effect could be easily detected using long data series of observations.
However, the oscillations have clear asymmetry, which we interpreted as
elliptical shape of the third components orbit. Parabolic trend we explained as
mass transfer between components of binary system. For all stars we computed
minimal possible mass of the third component. In addition, we developed our own
code in computed language Python and using it we computed orbital elements of
the third component. Moreover, for stars with parabolic trend we calculated
rate of the mass transfer. Finally, for all computed values we estimated
errors.
