We report the discovery of an eclipsing binary system with mass-ratio q $\sim$ 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 $85 \pm 4 M_{ \rm Jup}$ ($0.081M_{ \odot}$) and a radius of $0.84^{ +0.14}_{ -0.04} R_{ \rm Jup}$ ($0.084 R_{ \odot}$) that is comparable to that of Saturn. EBLM J0555-57Ab has a surface gravity $\log g_\mathrm{ 2} = 5.50^{ +0.03}_{ -0.13}$ and is one of the densest non-stellar-remnant objects currently known. These measurements are consistent with models of low-mass stars.
Categories: astro-ph.SR astro-ph.EP