Southern massive stars at high angular resolution. WR 25 is a massive hierarchical triple system

WR 25 is a massive colliding-wind binary in the Carina nebula comprising a WN6ha primary star with an O5 companion in an eccentric 208-day orbit. Recent spectroscopic analysis estimates the total binary mass to approach 100,M_⊙, and a primary-to-secondary mass ratio of q= M_1/M_2 hinting at a very massive primary star. The presence of additional spectroscopic signatures from a third, ``intruder'' star was also noted, making it a candidate hierarchical triple system. In this study, we present a VLTI/PIONIER interferometric observation of WR 25, spatially resolving all three components for the first time. For the inner WN6ha + O5 binary, we find a projected angular separation of 1.68 milliarcseconds (mas), corresponding to a projected physical separation of 3.90 au at a distance of 2.32 ± 0.12 kpc. Leveraging the fortunate timing of the VLTI observation, which was obtained when the two components were passing the line of nodes, we determined the orbital semi-major axis a=3.11 au. Subsequently, the newly constrained total dynamical binary mass is 93 with a primary mass M_1=62 M_⊙ and secondary mass M_2=31 ,M_⊙. Based on simulations, we estimate the tertiary period to be in the range 19 -- 82 years. The newly confirmed triple nature of WR 25 thus makes it an important benchmark system to measure accurate dynamical masses of the inner binary and potentially the tertiary, to calibrate stellar evolution and atmosphere models, and to study its formation and stability as a hierarchical triple system. M_⊙. We detect the third star or tertiary with a projected angular separation of 27.69 _ mas from the primary, with a chance alignment probability lower than 10^-4. Using newly obtained brightness ratios between all components, we revisit archival spectroscopic data of WR 25 to disentangle spectra for individual components and derive their stellar parameters. The tertiary, which has a spectral type O7, is broadly coeval with the inner binary and has an evolutionary mass M_3=25.6^ +2.8 -2.3