Orbital period investigation of some short-period sdB-type eclipsing binaries

To understand the properties of orbital period changes of four short period sdB-type eclipsing binaries, HW Vir, HS 0705 + 6700, HS 2231 + 2441, and NSVS 14256825, we have monitored them photometrically since 2006. Apart from HW Vir, it is found that the O-C diagrams of HS 0705 + 6700 and HS 2231 + 2441 can also be described by a cyclic variation superimposed on a downward parabolic curve, indicating a long-term period decrease in the two systems. Therefore, of the four binary systems, long-term period variations have been detected for three ones that can be explained by magnetic braking of the fully convective component stars. This may drive these systems to evolution into normal cataclysmic variables. As for the cyclic period variations observed in the four systems, the light-travel time effect via the presence of a tertiary component is the most possible mechanism to interpret the periodic change, which reveals the presence of extremely low-mass objects (e.g., brown dwarf stars and huge planets) in this kind of binaries.