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Tension remains between the observed and modeled properties of substellarobjects, but objects in binary orbits, with known dynamical masses can providea way forward. HD 72946 B is a recently imaged brown dwarf companion to thenearby, solar type star. We achieve $\sim100~\mu\mathrm{as}$ relativeastrometry of HD 72946 B in the K-band using VLTI/GRAVITY, unprecedented for abenchmark brown dwarf. We fit an ensemble of measurements of the orbit usingorbitize! and derive a strong dynamical mass constraint$\mathrm{M_B}=69.5\pm0.5~\mathrm{M_{Jup}}$ assuming a strong prior on the hoststar mass $\mathrm{M_A}=0.97\pm0.01~\mathrm{M_\odot}$ from an updated stellaranalysis. We fit the spectrum of the companion to a grid of self-consistentBT-Settl-CIFIST model atmospheres, and perform atmospheric retrievals usingpetitRADTRANS. A dynamical mass prior only marginally influences the sampleddistribution on effective temperature, but has a large influence on the surfacegravity and radius, as expected. The dynamical mass alone does not stronglyinfluence retrieved pressure-temperature or cloud parameters within our currentretrieval setup. Independent of cloud prescription and prior assumptions, wefind agreement within $\pm2\,\sigma$ between the C/O ratio of the host($0.52\pm0.05)$ and brown dwarf ($0.43$ to $0.63$), as expected from amolecular cloud collapse formation scenario, but our retrieved metallicitiesare implausibly high ($0.6-0.8$) in light of an excellent agreement of the datawith the solar abundance model grid. Future work on our retrieval frameworkwill seek to resolve this tension. Additional study of low surface-gravityobjects is necessary to assess the influence of a dynamical mass prior onatmospheric analysis.

VLTI/GRAVITY Observations and Characterization of the Brown Dwarf Companion HD 72946 B