The EBLM project

III. A Saturn-size low-mass star at the hydrogen-burning limit^⋆

¹ Cavendish Laboratory, J J Thomson Avenue, Cambridge, CB3 0HE, UK
e-mail: av478@cam.ac.uk
² Institute of Astronomy, Madingley Road, Cambridge, CB3 0HA, UK
³ Observatoire Astronomique de l’Université de Genève, Chemin des Maillettes 51, 1290 Sauverny, Switzerland
⁴ Astrophysics Group, Keele University, Staffordshire, ST55 BG, UK
⁵ Space Research Institute, Austrian Academy of Sciences, Schmiedlstr. 6, 8042 Graz, Austria
⁶ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁷ SUPA, School of Physics & Astronomy, University of St Andrews, North Haugh, KY16 9SS, St Andrews, Fife, UK
⁸ Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
⁹ Université de Liège, Allée du 6 août 17, Sart Tilman, 4000 Liège 1, Belgium
¹⁰ Instituto de Astronomía, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510 Ciudad de México, México
¹¹ Hobart and William Smith Colleges, Department of Physics, Geneva, NY 14456, USA

Received: 4 May 2017
Accepted: 12 June 2017

Abstract

We report the discovery of an eclipsing binary system with mass-ratio q ∼ 0.07. After identifying a periodic photometric signal received by WASP, we obtained CORALIE spectroscopic radial velocities and follow-up light curves with the Euler and TRAPPIST telescopes. From a joint fit of these data we determine that EBLM J0555-57 consists of a sun-like primary star that is eclipsed by a low-mass companion, on a weakly eccentric 7.8-day orbit. Using a mass estimate for the primary star derived from stellar models, we determine a companion mass of 85 ± 4 M_Jup (0.081 M_⊙) and a radius of 0.84^{+ 0.14}_-0.04R_Jup (0.084 R_⊙) that is comparable to that of Saturn. EBLM J0555-57Ab has a surface gravity log g₂ =5.50^{+ 0.03}_-0.13 and is one of the densest non-stellar-remnant objects currently known. These measurements are consistent with models of low-mass stars.