Light-Time Effect in the Orbital Period Variations of the Eclipsing Binaries TT Del, EU Hya, and SV Tau

Variations of the orbital period of the eclipsing binaries TT Del, EU Hya, and SV Tau have been studied. It has been shown that they can be represented by cyclic fluctuations without a secular change. Cyclic variations of the EU Hya and SV Tau periods can be explained by the light-time effect due to the presence of the third body in the system. The long period of EU Hya is 22.7 yr, 107.5 yr of SV Tau. The minimum masses of the extra bodies in these systems are 0.26 M-circle dot and 0.38 M-circle dot respectively. In TT Del, a superposition of two cyclic variations of the orbital period with the periods of 21.1 and 54.5 yr is present. Variations of the TT Del period can be explained both by the light-time effect and by magnetic cycles.