Detection and Astrometry of the Ba–Bb Subsystem in α Piscium: First Dual-field Interferometry at the CHARA Array

We present the first on-sky demonstration of dual-field interferometry at the Center for High Angular Resolution Astronomy (CHARA) Array and the first direct resolution of the inner Ba–Bb subsystem in the bright hierarchical triple α Piscium. Using H -band fringe tracking on component A with MIRC-X to stabilize K -band science fringes on component B with MYSTIC, we detected a companion at a projected separation of 7 mas, confirming a long-suspected but previously unresolved short-period subsystem within the B component. The nearly equal H / K -band flux ratio indicates that Ba and Bb are near-twin F-type stars, consistent with the two narrow-lined components seen in optical spectra of B. By combining CHARA interferometry with archival VLTI/GRAVITY astrometry and radial velocities from archival and new spectroscopy (NARVAL and ARCES), we derive a well-constrained orbit with a period of P = 25 days, eccentricity e ≃ 0.6, and inclination i ≃ 65°, yielding precise dynamical masses of 1.668 ± 0.033 M ⊙ and 1.646 ± 0.029 M ⊙ . No additional companion is detected down to Δ H ≈ 5 at separations of 0.2–2 au. We also obtained dual-field differential astrometry of the wide A–B pair with a precision of ∼0.234 mas at a separation of 1 . ″ 85, with an error budget dominated by internal delay-line actuators, fringe-tracking performance, and chromatic dispersion. While the long-period outer orbit is not refined by these measurements, their agreement with the published astrometric orbit provides an on-sky validation of the CHARA dual-field mode. These results establish α Psc as a well-characterized hierarchical system suitable for future benchmark studies and demonstrate CHARA’s new capability for off-axis interferometry and submilliarcsecond astrometry on arcsecond-scale binaries.