The EBLM project. III. A Saturn-size low-mass star at the hydrogen-burning limit

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.

The photometry tables and radial velocities are only available at the CDS and on demand via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/604/L6