Abstract
We present the new results of our long-term observational project to analyze the orbital period variations of low-mass eclipsing binaries. More then 200 new precise mid-eclipse times recorded with a CCD were obtained for three eclipsing binaries with short orbital periods: V380 Dra (P = 0(d).49), BX Tri (0(d).19), and V642 Vir (0(d). 52).
Observed-minus-calculated diagrams of the stars were analyzed using all reliable timings, and new parameters of the light-time effect were obtained. We derived for the first time the short orbital periods of possible third bodies of 10-20 years for these objects.
We calculated that the minimum masses of the third component are close to 0.2 M-circle dot, which corresponds to the mass of M4 - M5 red dwarfs. The multiplicity of these systems also plays an important role in the precise determination of their physical parameters.