The Spin-orbit alignment of five long period eclipsing binary systems

Abstract We present a joint analysis of TESS photometry and Minerva-Australis radial velocity data for five long-period eclipsing binary systems with late-F to early-G primaries: TIC 28051929, TIC 49899799, TIC 89045042, TIC 410314066, and TIC 446872386. All systems host early- to late-M dwarf companions that lie in “tropical” orbits (P ∼ 7-30 d) with eccentricities ranging from nearly circular to significantly eccentric (e ∼ 0.37). Sky-projected obliquities are derived for each system using a classical analysis of the radial velocity perturbations produced by the Rossiter-McLaughlin effect. Among the five systems, TIC 28051929 stands out as clearly misaligned, with a measured sky-projected obliquity of $\lambda _\mathrm{A} = -27.9^{+7.1}_{-5.4}\circ$, marking the first quantitative finding of significant misalignment in a binary system with an F-class primary. We did not find compelling evidence for misalignment in the remaining systems; however TIC 410314066 remains a strong candidate for follow-up, as our incomplete dataset hints at possible misalignment in this case. Taken together, these results expand the current limited sample of binary systems with quantitative obliquity measurements, particularly for systems with cooler primaries. While most binaries appear aligned, the significant misalignment in TIC 28051929 demonstrates that substantial spin-orbit tilts can persist even in systems where tidal realignment would otherwise be expected to act.