The Hot, Continuum-compact State of the Yellow Hypergiant V509 Cassiopeiae: CHARA Diameters, CO-band Angular-extension Constraints, and Companion Limits

Yellow hypergiants are rare, evolved massive stars generally interpreted as post-red-supergiant objects evolving blueward through or near the dynamically unstable Yellow Evolutionary Void. V509 Cassiopeiae is a remarkable example: its spectroscopically inferred effective temperature increased from approximately 5000 K in the mid-1980s to about 7900 K by the late 1990s and has since remained near this value during its post-heating state. We use simultaneous six-telescope CHARA observations obtained in 2023 with MIRC-X in the H band and MYSTIC in the K band to measure its photospheric diameter, search for a luminous companion, and constrain any spatially extended CO atmosphere. Interferometric data fitting with fixed atmosphere-model center-to-limb variation profiles yield limb-darkened angular diameters of 1.1840 -- 1.1950 mas. Although nominally about 5% smaller than the previous Palomar Testbed Interferometer measurement, the difference is only 1.8σ, indicating no significant secular change over 17--21 yr. The closure phases are consistent with a centrosymmetric brightness distribution, and no near-infrared-bright companion is detected over the sampled separations. The median 3σ limits are ΔH=5.28 mag and ΔK=6.48 mag. The B1 V companion proposed from ultraviolet spectroscopy is expected to lie below these limits and is not excluded. The CO first-overtone channels give a weighted mean CO-to-continuum diameter ratio of 1.011 plus or minus0.004, below the adopted 3σ detection criterion. We find no significant CO-band angular extension. V509 Cas is thus hot and molecularly compact despite retaining spectroscopically visible atomic circumstellar gas.