Pulsations and eclipse-time analysis of HWVir

Abstract

We analysed recent K2 data of the short-period eclipsing binary systemHWVir,which consists of a hot subdwarf-B type primary with an M-dwarf companion.We determined the mid-times of eclipses, calculated O–C diagrams, and an average shift of the secondary minimum. Our results show that the orbital period is stable within the errors over the course of the 70 days of observations. Interestingly, the offset from mid-orbital phase between the primary and the secondary eclipses is found to be 1.62 s. If the shift is explained solely by light-travel time, the mass of the sdB primary must be 0.26M , which is too low for the star to be core-helium burning. However, we argue that this result is unlikely to be correct and that a number of effects caused by the relative sizes of the stars conspire to reduce the effective light-travel time measurement. After removing the flux variation caused by the orbit, we calculated the amplitude spectrum to search for pulsations. The spectrum clearly shows periodic signal from close to the orbital frequency up to 4600 μHz, with the majority of peaks found below 2600 μHz. The amplitudes are below0.1 part-per-thousand, too lowto be detected with groundbased photometry. Thus, the high-precision data from the Kepler spacecraft has revealed that the primary of the HWVir system is a pulsating sdBV star. We argue that the pulsation spectrum of the primary in HWVir differs from that in other sdB stars due to its relatively fast rotation that is (nearly) phase-locked with the orbit.