RX Gru: A short-period pre-main-sequence eclipsing binary with a distant circumbinary companion

We report the discovery of a new short-period pre-main-sequence eclipsing binary, RX Gru, orbited by a distant circumbinary companion. We characterised the system by analysing the photometric observations from the Solaris network, the Transiting Exoplanet Survey Satellite , and the Super Wide Angle Search for Planets survey, combined with the radial velocities from four high-resolution spectrographs: HARPS, FEROS, CHIRON, and HRS. We derived the parameters of the eclipsing components, which are M Aa = 1.004 −0.026 +0.027 M ⊙ , R Aa = 1.007 ± 0.021 R ⊙ , and T eff, Aa = 5379 ± 289 K for the primary and M Ab = 0.985 −0.024 +0.025 M ⊙ , R Ab = 1.024 ± 0.023 R ⊙ , and T eff, Ab = 5322 ± 278 K for the secondary. We determined the age of the system from the observed parameters using two evolution codes, MESA and Cesam2k20. We obtained an age of ∼28 Myr, placing the two stars at the very end of the pre-main-sequence phase. We also derived the minimum mass and orbital period of the tertiary companion, which are found to be M B = 89.0 ± 3.5 M Jup and P AB = 2.379 −0.025 +0.10 yr, respectively. We conclude that RX Gru consists of a tight inner binary composed of two twin components and an outer low-mass companion (a massive brown dwarf or a very low mass star) in a relatively wide orbit, and we suggest that the system was formed via the dynamical unfolding mechanism coupled with the shared accretion of the circumbinary material by the binary components.