* Astronomy

Title: Orbital properties of an unusually low-mass sdB star in a close binary system with a white dwarf
Authors: R. Silvotti, R. H. Ostensen, S. Bloemen, J. H. Telting, U. Heber, R. Oreiro, M. D. Reed, L. E. Farris, S. J. O'Toole, L. Lanteri, P. Degroote, H. Hu, A. S. Baran, J. J. Hermes, L. G. Althaus, T. R. Marsh, S. Charpinet, J. Li, R. L. Morris, D. T. Sanderfer

We have used 605 days of photometric data from the Kepler spacecraft to study KIC 6614501, a close binary system with an orbital period of 0.15749747(25) days (3.779939 hours), that consists of a low-mass subdwarf B (sdB) star and a white dwarf. As seen in many other similar systems, the gravitational field of the white dwarf produces an ellipsoidal deformation of the sdB which appears in the light curve as a modulation at two times the orbital frequency. The ellipsoidal deformation of the sdB implies that the system has a maximum inclination of ~40 degrees, with i ~ 20° being the most likely. The orbital radial velocity of the sdB star is high enough to produce a Doppler beaming effect with an amplitude of 432 ±5 ppm, clearly visible in the folded light curve. The photometric amplitude that we obtain, K1 = 85.8 km/s, is ~ 12 per cent less than the spectroscopic RV amplitude of 97.2 ±2.0 km/s. The discrepancy is due to the photometric contamination from a close object at about 5 arcsec North West of KIC 6614501, which is difficult to remove. The atmospheric parameters of the sdB star, Teff = 23 700 ±500 K and log g = 5.70 ±0.10, imply that it is a rare object below the Extreme Horizontal Branch (EHB), similar to HD 188112 (Heber et al. 2003). The comparison with different evolutionary tracks suggests a mass between ~ 0.18 and ~ 0.25 solar masses, too low to sustain core helium burning. If the mass was close to 0.18-0.19 solar masses, the star could be already on the final He-core WD cooling track. A higher mass, up to ~0.25 solar masses, would be compatible with a He-core WD progenitor undergoing a cooling phase in a H-shell flash loop. A third possibility, with a mass between ~0.32 and ~0.40 solar masses, can not be excluded and would imply that the sdB is a "normal" (but with an unusually low mass) EHB star burning He...

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