The EBLM Project XVIII. 3D Obliquities of Five Low-Mass Eclipsing Binaries

TL;DR

This study expands the measured obliquities of short-period eclipsing binaries by deriving five new true 3D obliquities ($\psi$) from Rossiter–McLaughlin measurements, primary-star rotation, and joint RV+photometry fitting. Using CORALIE/HARPS RVs and TESS light curves, the authors perform a joint Bayesian analysis to extract orbital parameters, masses, radii, and the RM signal, recovering sky-projected obliquities $|\lambda|$ near zero for most targets while obtaining $\psi$ values typically $<20^{\circ}$. The results reveal mild spin–orbit misalignments in some systems, especially for primaries above the Kraft break, and show that M-dwarf radii are inflated by more than $5\sigma$ relative to MIST models for four of the five stars, highlighting persistent gaps in low-mass stellar structure models and the complexity of tidal realignment in binaries with high mass ratios. Together, these findings provide important constraints on binary formation and tidal evolution theories and demonstrate the value of combining RM, asteroseismic-like rotation information from spot modulation, and precise stellar modeling. The work substantially increases the sample of binary obliquities with true 3D measurements, offering new opportunities to test tidal theory across different mass ratios and evolutionary states.

Abstract

A question that continues to perplex astronomers is the formation of tight stellar binaries. There is too much angular momentum in a collapsing and fragmenting protostellar cloud to form a stellar binary in situ with a separation less than an AU, yet thousands of these short-period binaries have been discovered. One indication of a binary's formation is the angle between the stellar spin and orbital axes -- its obliquity. The classical method for determining projtected stellar obliquity is the Rossiter-McLaughlin effect. This method has been applied to over 100 hot Jupiters, yet only a handful of stellar binaries. Of the binary systems with measured projected obliquities, even fewer have measured 3D obliquities. In this paper, we add five more short-period binary 3D obliquity measurements to the sample that previously consisted of a single system. We present Rossiter-McLaughlin measurements for EBLM J0239-20, EBLM J0941-31, EBLM 1037-25, EBLM 1141-37, and EBLM J2025-45. These systems consist of an M-dwarf eclipsing an F/G type primary. We combined CORALIE and HARPS spectroscopy with TESS photometry of primary and secondary eclipses. We show that even though the sky-projected obliquities seem to be aligned, there is modest but non-zero spin-orbit misalignment ($ψ$ between 5 and 20$^{\circ}$). Our primary stars straddle the Kraft break at $\sim 6250K$. Finally, we derive the M-dwarf masses and radii to precisions better than 3\%. With the exception of EBLM J0941-31, each system has an inflated radius greater than $5σ$ from the expected radius from stellar models.

Figures (11)

• Figure 1:
• Figure 2: Orbital geometry of stellar obliquities adapted from a figure in Albrecht2021. The diagram shows $\psi$ - the true stellar obliquity, $\lambda$ - the sky-projected spin-orbit angle, $I_{\rm \star}$ - the inclination of the rotation axis of the primary star, and $I_{\rm bin}$ - the orbital inclination of the binary.
• Figure 3: Sky-projected stellar obliquity $\rm |\lambda|$ vs. mass ratio (q, left) and true 3-D Stellar Obliquities ($\psi$) vs mass ratio (right) for eclipsing companions, $M_b > 0.7$$M_{\rm J}$. Obliquities measured via apsidal motion are not depicted. Red points depict systems with primary stars below the Kraft break Kraft1967, and blue points depict those above. The new systems we add are shown as diamonds. The giant planet and brown dwarf obliquities were retrieved from TEPCat Southworth2011 and Carmichael2025Vowell2025b. The eclipsing binary obliquities are from Marcussen2022Spejcher2025bWells2025.
• Figure 4: Mass vs. Radius plot for high-precision M-dwarfs, with data taken from Maxted2023. The plot includes targets from the EBLM survey (pink dots) and general literature targets (blue dots). Only targets with a mass or radius better than 10% are included here. The solid line passing through the data points indicates the MIST stellar model for [Fe/H] = 0.0. Our systems are highlighted by the black markers.
• Figure 5: