The EBLM project – XVI. Moderate spin–orbit misalignment of the low-mass eclipsing binary EBLM J0021-16

ABSTRACT Thousands of tight ($< 1$ au) main-sequence binaries have been discovered, but it is uncertain how they formed. There is likely too much angular momentum in a collapsing fragmenting protostellar cloud to form such binaries in situ, suggesting some post-processing. One probe of a binary’s dynamical history is the angle between the stellar spin and orbital axes – its obliquity. The classical method for determining stellar obliquity is the Rossiter–McLaughlin effect. It has been applied to over 100 hot Jupiters, but less than a dozen stellar binaries. In this paper, we present the Rossiter–McLaughlin measurement of EBLM J0021-16, a $0.19M_\odot$ M-dwarf eclipsing a $1.05M_\odot$ G-dwarf on a 5.97-d almost-circular orbit. We combine CORALIE spectroscopy with TESS photometry and a measured primary star rotation period of 7.04 d, according to starspot modulation. We show that the orbital axis is misaligned with the primary star’s spin axis, with a true 3D obliquity of $\psi =28.9\pm 2.1^{\circ }$. EBLM J0021-16, being neither spin–orbit aligned nor synchronized, yet with an almost circular orbit, is a curious case for tidal evolution in tight binaries. It becomes one of a handful of eclipsing binaries with true obliquity measurements. Finally, we derive the M-dwarf’s mass and radius to a fractional precision better than 1 per cent. The radius of the M-dwarf is inflated by 6 per cent ($7.4\sigma$) with respect to stellar models, consistent with many other M-dwarfs in the literature.