Context. Interferometry can provide spatially resolved observations of massive star binary systems and their colliding winds, which thus far have been studied mostly with spatially unresolved observations. Aims. We present the first AMBER/VLTI observations, taken at orbital phase 0.32, of the Wolf-Rayet and O (WR+O) star binary system γ 2 Velorum and use the interferometric observables to constrain its properties. Methods. The AMBER/VLTI instrument was used with the telescopes UT2, UT3, and UT4 on baselines ranging from 46 m to 85 m. It delivered spectrally dispersed visibilities, as well as differential and closure phases, with a resolution R = 1500 in the spectral band 1.95−2.17 µm. We interpret these data in the context of a binary system with unresolved components, neglecting in a first approximation the wind-wind collision zone flux contribution. Results. Using WR-and O-star synthetic spectra, we show that the AMBER/VLTI observables result primarily from the contribution of the individual components of the WR+O binary system. We discuss several interpretations of the residuals, and speculate on the detection of an additional continuum component, originating from the free-free emission associated with the wind-wind collision zone (WWCZ), and contributing at most to the observed K-band flux at the 5% level. Based on the accurate spectroscopic orbit and the Hipparcos distance, the expected absolute separation and position angle at the time of observations were 5.1 ± 0.9 mas and 66 ± 15• , respectively. However, using theoretical estimates for the spatial extent of both continuum and line emission from each component, we infer a separation of 3.62 +0.11 −0.30 mas and a position angle of 73• , compatible with the expected one. Our analysis thus implies that the binary system lies at a distance of 368 +38 −13 pc, in agreement with recent spectrophotometric estimates, but significantly larger than the Hipparcos value of 258 +41 −31 pc.

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Context. Young and close multiple systems are unique laboratories to probe the initial dynamical interactions between forming stellar systems and their dust and gas environment. Their study is a key building block to understanding the high frequency of main-sequence multiple systems. However, the number of detected spectroscopic young multiple systems that allow dynamical studies is limited. GW Orionis is one such system. It is one of the brightest young T Tauri stars and is surrounded by a massive disk. Aims. Our goal is to probe the GW Orionis multiplicity at angular scales at which we can spatially resolve the orbit. Methods. We used the IOTA/IONIC3 interferometer to probe the environment of GW Orionis with an astronomical unit resolution in 2003, 2004, and 2005. By measuring squared visibilities and closure phases with a good UV coverage we carry out the first image reconstruction of GW Ori from infrared long-baseline interferometry. Results. We obtained the first infrared image of a T Tauri multiple system with astronomical unit resolution.We show that GW Orionis is a triple system, resolve for the first time the previously known inner pair (separation ρ ∼ 1.4 AU) and reveal a new more distant component (GW Ori C) with a projected separation of ∼8 AU with direct evidence of motion. Furthermore, the nearly equal (2:1) H-band flux ratio of the inner components suggests that either GW Ori B is undergoing a preferential accretion event that increases its disk luminosity or that the estimate of the masses has to be revisited in favour of a more equal mass-ratio system that is seen at lower inclination. Conclusions. Accretion disk models of GW Ori will need to be completely reconsidered because of this outer companion C and the unexpected brightness of companion B.