Discovery of a Stripped Red-giant Core in a Bright Eclipsing Binary Star

We report the serendipitous discovery from WASP archive photometry of a binary star in which an apparently normal A-type star (J0247-25 A) eclipses a smaller, hotter subdwarf star (J0247-25 B). The kinematics of J0247-25 A show that it is a blue-straggler member of the Galactic thick-disk. We present follow-up photometry and spectroscopy from which we derive approximate values for the mass, radius and luminosity for J0247-25 B assuming that J0247-25 A has the mass appropriate for a normal thick-disk star. We find that the properties of J0247-25 B are well matched by models for a red giant stripped of its outer layers and currently in a shell hydrogen-burning stage. In this scenario, J0247-25 B will go on to become a low mass white dwarf (M similar to 0.25M(circle dot)) composed mostly of helium. J0247-25 B can be studied in much greater detail than the handful of pre helium white dwarfs (pre-He-WD) identified to-date. These results have been published by Maxted et al. (2011). We also present a preliminary analysis of more recent observations of J0247-25 with the UVES spectrograph, from which we derive much improved masses for both stars in the binary. We find that both stars are more massive than expected and that J0247-25 A rotates sub-synchronously by a factor of about 2. We also present lightcurves for 5 new eclipsing pre-He-WD subsequently identified from the WASP archive photometry, 4 of which have mass estimates for the subdwarf companion based on a pair of radial velocity measurements.