  HS 0705+6700 is a short-period (P=2.3 hours), close binary containing a hot
sdB-type primary and a fully convective secondary. We have monitored this
eclipsing binary for more than 2 years and as a result, 32 times of light
minimum were obtained. Based on our new eclipse times together with these
compiled from the literature, it is discovered that the O-C curve of HS
0705+6700 shows a cyclic variation with a period of 7.15 years and a
semiamplitude of 92.4 s. The periodic change was analyzed for the light-travel
time effect that may be due to the presence of a tertiary companion. The mass
of the third body is determined to be M3 sin i = 0.0377 (+/-0.0043) Msun when a
total mass of 0.617 Msun for HS 0705+6700 is adopted. For orbital inclinations
i >= 32.8, the mass of the tertiary component would be below the stable
hydrogen-burning limit of M3~0.072 Msun, and thus it would be a brown dwarf.
The third body is orbiting the sdB-type binary at a distance shorter than 3.6
astronomical units (AU). HS 0705+6700 was formed through the evolution of a
common envelope after the primary becomes a red giant. The detection of a
sub-stellar companion in HS 0705+6700 system at this distance from the binary
could give some constraints on stellar evolution in such systems and the
interactions between red giants and their companions.

Solar and Stellar Astrophysics