; Context.; β; Pictoris is arguably one of the most studied stellar systems outside of our own. Some 30 yr of observations have revealed a highly-structured circumstellar disk, with rings, belts, and a giant planet:; β; Pictoris b. However very little is known about how this system came into being.; ; ; Aims.; Our objective is to estimate the C/O ratio in the atmosphere of; β; Pictoris b and obtain an estimate of the dynamical mass of the planet, as well as to refine its orbital parameters using high-precision astrometry.; ; ; Methods.; We used the GRAVITY instrument with the four 8.2 m telescopes of the Very Large Telescope Interferometer to obtain; K; -band spectro-interferometric data on; β; Pic b. We extracted a medium resolution (; R; = 500); K; -band spectrum of the planet and a high-precision astrometric position. We estimated the planetary C/O ratio using two different approaches (forward modeling and free retrieval) from two different codes (ExoREM and petitRADTRANS, respectively). Finally, we used a simplified model of two formation scenarios (gravitational collapse and core-accretion) to determine which can best explain the measured C/O ratio.; ; ; Results.; Our new astrometry disfavors a circular orbit for; β; Pic b (; e; = 0.15; −0.04; +0.05; ). Combined with previous results and with H; IPPARCOS; /; Gaia; measurements, this astrometry points to a planet mass of; M; = 12.7 ± 2.2; M; Jup; . This value is compatible with the mass derived with the free-retrieval code petitRADTRANS using spectral data only. The forward modeling and free-retrieval approches yield very similar results regarding the atmosphere of; β; Pic b. In particular, the C/O ratios derived with the two codes are identical (0.43 ± 0.05 vs. 0.43; −0.03; +0.04; ). We argue that if the stellar C/O in; β; Pic is Solar, then this combination of a very high mass and a low C/O ratio for the planet suggests a formation through core-accretion, with strong planetesimal enrichment.;
