VLTI/GRAVITY enables the determination of the first dynamical masses of a classical Be + stripped and bloated pre-subdwarf binary

Context. HR 6819 is the first post-mass transfer binary system composed of a classical Be star and a bloated pre-subdwarf stripped star directly confirmed by interferometry. While the Be star is already spun up to near-critical rotation and possesses a self-ejected viscous Keplerian disk, the stripped star is found in a short-lived evolutionary stage, in which it retains the spectral appearance of a B-type main-sequence star while contracting into a faint subdwarf OB-type star. Aims. In order to understand the evolution of intermediate-mass interacting binaries, the fundamental parameters of cornerstone objects such as HR 6819 need to be known. We aim to obtain orbital parameters and model-independent dynamical masses of this binary system to quantitatively characterize this rarely observed evolutionary stage. Methods. We analyzed a time series of 12 interferometric near-IR K-band observations from VLTI/GRAVITY with the help of the geometrical model-fitting tool PMOIRED. We included recently published radial velocities based on FEROS high-resolution spectroscopy for the binary orbital solution. Results. With the GRAVITY data, we obtained the astrometric orbit, relative fluxes of the components, and parameters of the circumstellar disk of the Be star; we also detected helium line signatures from the stripped star. Using the published radial velocities enabled us to obtain the dynamical masses of the components as well as the dynamical parallax. The Be star is the slightly brighter component in the K band and is almost 16 times as massive as the bloated stripped star, with the individual dynamical masses being 4.24±0.31 M_⊙ for the Be star and 0.270±0.056 M_⊙ for the stripped star. The orbit is slightly eccentric, with e = 0.0289±0.0058, and the semimajor axis of the orbit is 0.3800±0.0093 AU. The distance derived from the orbital solution is 296.0±8.0 pc, significantly lower than the distance from Gaia DR3, which is overestimated by ∼24% due to the orbital motion. Conclusions. The newly obtained fundamental parameters provide an important anchor for evolutionary models of interacting binaries and for the physics of mass transfer. The low mass of the bloated star means that it may become completely undetectable once it settles into a faint subdwarf, which implies that many more Be stars may have low-mass companions despite appearing single.