  The first complete orbital solution for the double-lined spectroscopic binary
system \gamma^2 Velorum, obtained from measurements with the Sydney University
Stellar Interferometer (SUSI), is presented. This system contains the closest
example of a Wolf-Rayet star and the promise of full characterisation of the
basic properties of this exotic high-mass system has subjected it to intense
study as an archetype for its class. In combination with the latest
radial-velocity results, our orbital solution produces a distance of
336^{+8}_{-7} pc, significantly more distant than the Hipparcos estimation
(Schaerer et al. 1997; van der Hucht 1997). The ability to fully specify the
orbital parameters has enabled us to significantly reduce uncertainties and our
result is consistent with the VLTI observational point (Millour et al. 2006),
but not with their derived distance. Our new distance, which is an order of
magnitude more precise than prior work, demands critical reassessment of all
distance-dependent fundamental parameters of this important system. In
particular, membership of the Vela OB2 association has been reestablished, and
the age and distance are also in good accord with the population of young stars
reported by Pozzo et al. (2000). We determine the O-star primary component
parameters to be M_V(O) = -5.63 \pm 0.10 mag, R(O) = 17 \pm 2 R_{\sun} and
{\cal M}(O) = 28.5 \pm 1.1 M_{\sun}. These values are consistent with
calibrations found in the literature if a luminosity class of II--III is
adopted. The parameters of the Wolf-Rayet component are M_v(WR) = -4.33 \pm
0.17 mag and {\cal M}(WR) = 9.0 \pm 0.6 M_{\sun}.

Astrophysics