High-speed photometry of the pre-cataclysmic binary HW Virginis and its orbital period change

The broad band B and V light curves of the pre-cataclysmic eclipsing binary HW Vir were obtained. All the available eclipse timings, including the new ones, spanning 19 years were analyzed under the third-body hypothesis. The residuals between the observed and calculated times of mid-eclipse show a long-term sinusoidal variation. The analysis yields the parameters of the third-body orbit, as well as limiting mass for the tertiary object. The result of this analysis gives a light-time semi-amplitude of 112 s, an orbital period of 18.8 yr and an eccentricity of 0.12. The mass of the third star is below the theoretical threshold of 0.07 $M_\odot$ for a hydrogen burning star. Its minimum mass is about 0.022 $M_\odot$ and for a wide range of inclinations of third-body orbit, i.e. $i \ge19^\circ$, the mass is smaller than 0.07 $M_\odot$. Therefore, we suggest that the third star may be a brown dwarf candidate. Combining the semi-amplitudes of the radial velocities and the photometric light curve solution has allowed us to model the short-period detached binary HW Vir. The luminosity and radius of the dM companion are slightly larger than that given by low-mass models