We studied 9 close binary stellar systems in which one component eclipses another moving along its orbit. We found the increase or decrease of the period between eclipses in 6 systems. These changes can be caused by mass transfer between components. We calculated the rates of the mass transfer. Also, we found periodic changes of the period for 6 systems (in 3 systems both types of changes were superimposed). These changes may be explained by the presence of the third component in the system which makes the close binary system rotating around the common center of mass that lead to delays in moments of eclipses due to the finite speed of light. For these systems we estimated the minimal mass of the possible third components and for one system we obtained the parameters of its orbit.
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