摘要：Aims. As part of our long-term observational project we aim to measure very precise mid-eclipse times for low-mass eclipsing binaries, which are needed to accurately determine their period changes. Over two hundred new precise times of minimum light recorded with CCD were obtained for three eclipsing binaries with short orbital periods: NSVS 01286630 (\hbox{$P = 0\fd38$}), NSVS 02502726 (\hbox{$0\fd56$}), and NSVS 07453183 (\hbox{$0\fd37$}).
Methods. O−C diagrams of studied stars were analysed using all reliable timings, and new parameters of the light-time effect were obtained.
Results. We derived for the first time or improved the very short orbital periods of third bodies of between one and seven years for all measured low-mass systems. We calculated that the lowest masses of the third components are between those of red and brown dwarfs. The multiplicity of these systems also plays an important role in the precise determination of their physical parameters. This research is part of an ongoing collaboration between professional astronomers and the Czech Astronomical Society, Variable Star and Exoplanet Section.
