RX Gru: Young Pre-MS Triple System

• RX Gru is a young hierarchical triple system featuring a short-period pre-main-sequence eclipsing binary and a tertiary near the hydrogen-burning limit.
• High-precision photometry and spectroscopy enabled detailed measurements of stellar masses, radii, and orbital parameters, revealing an inner period of 0.74 days.
• The system serves as a benchmark for studying binary formation, illustrating the role of dynamical unfolding and shared accretion in young multiples.

Searching arXiv for RX Gru and closely related system/context papers. RX Gru is a young hierarchical triple system centered on a short-period pre-main-sequence eclipsing binary and a distant circumbinary companion. It was reported as a new short-period pre-main-sequence eclipsing binary orbited by a distant circumbinary companion, with the system characterized from photometric observations from the Solaris network, the Transiting Exoplanet Survey Satellite, and the Super Wide Angle Search for Planets survey, together with radial velocities from HARPS, FEROS, CHIRON, and HRS (Marcadon et al., 21 Sep 2025). The inner pair consists of two near-twin solar-type stars at the very end of the pre-main-sequence phase, while the tertiary lies near the hydrogen-burning limit and is described as either a massive brown dwarf or a very low-mass star.

1. Discovery and system classification

RX Gru was identified as a short-period, double-lined eclipsing binary embedded in a wider triple hierarchy. The characterization combined time-series photometry and high-resolution spectroscopy, allowing simultaneous inference of stellar dimensions, effective temperatures, radial-velocity amplitudes, and the properties of the outer orbit. The system is explicitly described as consisting of a tight inner binary composed of two twin components and an outer low-mass companion in a relatively wide orbit (Marcadon et al., 21 Sep 2025).

The classification of RX Gru as a pre-main-sequence system is central to its significance. The two eclipsing stars have masses close to $1\,\mathrm{M}_\odot$, yet their evolutionary state places them at the end of pre-main-sequence contraction rather than on a mature main sequence. This makes RX Gru a rare, well-characterized pre-main-sequence triple and, as presented in the source, a useful laboratory for studying the formation and early evolution of short-period binaries in hierarchical multiples.

2. Inner eclipsing binary

The inner binary comprises the primary Aa and the secondary Ab. Their measured parameters are closely matched, motivating the description of the pair as “twin” components.

Parameter	Aa	Ab
Mass	$1.004^{+0.027}_{-0.026}\,\mathrm{M}_\odot$	$0.985^{+0.024}_{-0.025}\,\mathrm{M}_\odot$
Radius	$1.007\pm0.021\,\mathrm{R}_\odot$	$1.024\pm0.023\,\mathrm{R}_\odot$
$T_{\rm eff}$	$5379\pm289\,\mathrm{K}$	$5322\pm278\,\mathrm{K}$
$\log g$	$4.433^{+0.018}_{-0.017}$	$1.004^{+0.027}_{-0.026}\,\mathrm{M}_\odot$0
$1.004^{+0.027}_{-0.026}\,\mathrm{M}_\odot$1	$1.004^{+0.027}_{-0.026}\,\mathrm{M}_\odot$2	$1.004^{+0.027}_{-0.026}\,\mathrm{M}_\odot$3
$1.004^{+0.027}_{-0.026}\,\mathrm{M}_\odot$4	$1.004^{+0.027}_{-0.026}\,\mathrm{M}_\odot$5	$1.004^{+0.027}_{-0.026}\,\mathrm{M}_\odot$6
$1.004^{+0.027}_{-0.026}\,\mathrm{M}_\odot$7	66.99 km/s (fixed from BF)	72.64 km/s (fixed)

Errors are stated as $1.004^{+0.027}_{-0.026}\,\mathrm{M}_\odot$8, and the component assignment is based on minor but significant temperature and radius differences (Marcadon et al., 21 Sep 2025).

The orbital solution shows an extremely compact binary. The orbital period is $1.004^{+0.027}_{-0.026}\,\mathrm{M}_\odot$9 days, approximately $0.985^{+0.024}_{-0.025}\,\mathrm{M}_\odot$0 hr, with eccentricity $0.985^{+0.024}_{-0.025}\,\mathrm{M}_\odot$1, indicating a near-circular orbit. The mass ratio is $0.985^{+0.024}_{-0.025}\,\mathrm{M}_\odot$2, the semimajor axis is $0.985^{+0.024}_{-0.025}\,\mathrm{M}_\odot$3, and the inclination is $0.985^{+0.024}_{-0.025}\,\mathrm{M}_\odot$4. The radial-velocity semi-amplitudes are $0.985^{+0.024}_{-0.025}\,\mathrm{M}_\odot$5 km/s and $0.985^{+0.024}_{-0.025}\,\mathrm{M}_\odot$6 km/s. The source emphasizes that the system exhibits clear SB2 eclipses, which underpins the robustness of the parameter determination (Marcadon et al., 21 Sep 2025).

3. Tertiary companion and hierarchical architecture

The tertiary companion was identified through eclipse timing variations. Its inferred minimum mass is $0.985^{+0.024}_{-0.025}\,\mathrm{M}_\odot$7, equivalent to $0.985^{+0.024}_{-0.025}\,\mathrm{M}_\odot$8, and its orbital period is $0.985^{+0.024}_{-0.025}\,\mathrm{M}_\odot$9 yr, or $1.007\pm0.021\,\mathrm{R}_\odot$0 days. The outer eccentricity is $1.007\pm0.021\,\mathrm{R}_\odot$1, also nearly circular (Marcadon et al., 21 Sep 2025).

These values place the tertiary near the hydrogen-burning limit. Accordingly, it is described as a massive brown dwarf or a very low-mass star rather than being assigned unambiguously to one category. The mass estimate is based on the light-travel time effect in the eclipse timing variations, using the measured ETV amplitude $1.007\pm0.021\,\mathrm{R}_\odot$2 and the outer period $1.007\pm0.021\,\mathrm{R}_\odot$3.

A notable architectural feature is the scale separation between the inner and outer orbits. RX Gru is reported to have the largest outer-to-inner period ratio known among pre-main-sequence triples, $1.007\pm0.021\,\mathrm{R}_\odot$4 (Marcadon et al., 21 Sep 2025). This extreme hierarchy is significant because it constrains viable formation channels: the system contains an exceptionally tight inner binary together with a much wider tertiary, yet all three components are inferred to be young.

4. Distance and evolutionary state

The age of RX Gru was determined from the observed masses, radii, and effective temperatures of the eclipsing components using two stellar evolution codes, MESA and Cesam2k20. The resulting age is approximately $1.007\pm0.021\,\mathrm{R}_\odot$5 Myr, with an uncertainty of about $1.007\pm0.021\,\mathrm{R}_\odot$6–$1.007\pm0.021\,\mathrm{R}_\odot$7 Myr, placing both stars at the very end of the pre-main-sequence phase. The measured parameters of Aa and Ab are independently consistent with a common system age, and both components match 27–29 Myr isochrones within $1.007\pm0.021\,\mathrm{R}_\odot$8 at solar metallicity (Marcadon et al., 21 Sep 2025).

The distance estimate derived from photometric parallax, using the measured $1.007\pm0.021\,\mathrm{R}_\odot$9, radii, and $1.024\pm0.023\,\mathrm{R}_\odot$0-magnitude, is $1.024\pm0.023\,\mathrm{R}_\odot$1 pc, corresponding to $1.024\pm0.023\,\mathrm{R}_\odot$2 mas. The Gaia DR3 parallax is $1.024\pm0.023\,\mathrm{R}_\odot$3 mas. The two values are stated to be not in agreement, with the discrepancy attributed in the source to likely bias from orbital motion not accounted for in the Gaia solution (Marcadon et al., 21 Sep 2025).

The evolutionary interpretation is therefore unusually specific for so young and so compact a binary. RX Gru is not merely a youthful eclipsing pair; it is a system in which both stars are inferred to be nearly coeval, nearly equal in mass, and already compressed into a $1.024\pm0.023\,\mathrm{R}_\odot$4-day orbit while still at the end of pre-main-sequence contraction.

5. Photometric variability and timing behavior

Both stars show substantial stellar activity. The source describes large, migrating cold spots that modulate the out-of-eclipse light curves and produce residual structure in the eclipse timing variations. The ETV residuals show a period of approximately $1.024\pm0.023\,\mathrm{R}_\odot$5–$1.024\pm0.023\,\mathrm{R}_\odot$6 days, interpreted as likely tied to spot migration and/or differential rotation (Marcadon et al., 21 Sep 2025).

This activity matters for two reasons. First, it affects the morphology of the photometric signal outside eclipse, which is relevant for light-curve modeling. Second, it introduces astrophysical timing structure that must be separated from the long-term LTTE signal used to infer the tertiary companion. A plausible implication is that RX Gru offers a case in which stellar activity, eclipse timing, and orbital dynamics must be modeled jointly rather than in isolation.

6. Formation scenario and broader significance

The proposed formation pathway for RX Gru is the dynamical unfolding mechanism coupled with shared accretion of circumbinary material by the binary components. In the source summary, the sequence is presented in three stages: initial formation as an unstable, non-hierarchical multiple; early dynamical interactions that reconfigure the system into a tight inner binary and a wide-orbit tertiary; and subsequent accretion and sharing of circumbinary gas by the close binary, leading to mass equalization and further orbital tightening (Marcadon et al., 21 Sep 2025).

Within that interpretation, the near-unity mass ratio of the inner pair is not incidental. The system’s “twin” character, $1.024\pm0.023\,\mathrm{R}_\odot$7, is treated as consistent with shared accretion after the onset of hierarchical structure. The outer object, meanwhile, remains in a wide, nearly circular orbit. The source further states that this pathway is strongly favored over Kozai-Lidov or tidal evolution scenarios for such a very young, nearly coplanar triple, because those alternative channels act on longer timescales and are inefficient in this regime.

RX Gru is therefore presented as a benchmark system for close-binary formation in young triples. Its importance derives from the conjunction of several properties that are rarely established simultaneously: precise masses and radii from eclipses and radial velocities; a pre-main-sequence age of $1.024\pm0.023\,\mathrm{R}_\odot$8 Myr; an inner period of $1.024\pm0.023\,\mathrm{R}_\odot$9 days; and a tertiary near the stellar-substellar boundary on a $T_{\rm eff}$0-yr orbit (Marcadon et al., 21 Sep 2025). This suggests that RX Gru is particularly informative for evaluating models in which early few-body dynamics and accretion jointly produce extremely close, nearly equal-mass binaries within hierarchical multiple systems.