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Abstract For nearly a decade, observations have shown that many older Sun-like stars spin faster than predicted, a phenomenon known as weakened magnetic braking (WMB). The leading hypothesis for WMB is a weakening of the large-scale dipole field, which leads to a less efficient angular momentum loss. To test this hypothesis on a star known to be in the WMB regime, we present the first Zeeman Doppler imaging (ZDI) map of the Sun-like starτCeti, reconstructed using spectropolarimetric data from the Canada–France–Hawai‘i Telescope. Our ZDI analysis reveals a remarkably simple, stable, and weak (〈B〉 = 0.17 G) magnetic field, characterized by a predominantly dipolar (∼92% magnetic energy contained inl= 1 modes), and highly axisymmetric (∼88% magnetic energy contained inm < l/2 modes) morphology. We infer a dipole field strength ofB_dip = 0.31 G, nearly an order of magnitude weaker than standard braking model predictions, providing direct confirmation of the weakened large-scale dipole predicted by the WMB hypothesis. This work establishes a new benchmark for ZDI, demonstrating that even extremely quiet stars in the WMB regime are accessible to this technique. 

Abstract Gyrochronology, a valuable tool for determining ages of low-mass stars where other techniques fail, relies on accurate calibration. We present a sample of 185 wide (>100 au) white dwarf + main sequence (WD + MS) binaries. Total ages of WDs are computed using all-sky survey photometry, Gaia parallaxes, and WD atmosphere models. Using a magnetic braking law calibrated against open clusters, along with assumptions about initial conditions and angular momentum transport, we construct gyrochrones to predict the rotation periods of MS stars. Both data and models show that, at the fully convective boundary (FCB), MS stars with WD ages of up to 7.5 Gyr and within a <50 K effective temperature range experience up to a threefold increase in rotation period relative to stars slightly cooler than the FCB. We suggest that rapid braking at this boundary is driven by a sharp rise in the convective overturn timescale (τ_cz) caused by structural changes between partially and fully convective stars and the³He instability occurring at this boundary. While the specific location in mass (or temperature) of this feature varies with model physics, we argue that its existence remains consistent. Stars along this feature exhibit rotation periods that can be mapped, within 1σ, to a range of gyrochrones spanning ≈6 Gyr. Due to current temperature errors (≃50 K), this implies that a measured rotation period cannot be uniquely associated to a single gyrochrone, implying that gyrochronology may not be feasible for M dwarfs very close to the FCB.