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\begin{document}
\title{FUNDAMENTAL STELLAR PARAMETERS OF \protect\boldmath{$\gamma^2$}~Vel\\
~\\
FROM HIPPARCOS DATA\/\thanks{Based on data from the ESA Hipparcos 
astrometry satellite.}}
\author{Daniel Schaerer\/\thanks{Observatoire de Gen\`eve, CH-1290 Sauverny, 
Switzerland.}\morethanks{Observatoire Midi-Pyr\'en\'ees, 14, Av.\ E.~Belin, 
F-31400 Toulouse, France.}\\
\\
Space Telescope Science Institute\\
\\
3700 San Martin Drive\\
\\
Baltimore, MD 21218\\
\and
Werner Schmutz\\
\\
Institut f\"ur Astronomie\\
\\
ETH-Zentrum\\
\\
CH-8092 Z\"urich, Switzerland\\
\and
Michel Grenon\\
\\
Observatoire de Gen\`eve\\
\\
CH-1290 Sauverny, Switzerland}

\tobe{1 August 1997}{ApJ (Letters)}
\recacc{11 March 1997}{22 May 1997}
\maketitle
\begin{abstract}

We report parallax measurements by the HIPPARCOS satellite of 
$\gamma^2$ Velorum and few related objects: The distance of 
$\gamma^2$~Vel is $d=258^{+41}_{-31}$~pc, significantly smaller 
than the commonly adopted value to Vela OB2.  For $\zeta$~Pup 
$d=429^{+120}_{-77}$~pc is in agreement with the canonical 
distance.

The total mass of $\gamma^2$ Vel derived from its parallax, the 
angular size of the semi-major axis as measured with intensity 
interferometry, and the period is $M(WR+O)=29.5\pm15.9M_{\odot}$.  
This result favors the orbital solution of Pike \etal. (1983) 
over that of Moffat \etal (1986).  The stellar parameters for the 
O~star companion derived from line blanketed non-LTE atmosphere 
models are: T$_{\rm eff}=34000\pm1500$~K, log $L/L_{\odot}=5.3\pm0.15$ 
from which an evolutional mass of $M=29\pm4M_{\odot}$ and an age of 
$4.0^{+0.8}_{-0.5}$ Myr is obtained from single star evolutionary 
models.  With non-LTE model calculations including He and C we 
derive a luminosity log $L/L_{\odot}\sim4.7\pm0.2$ for the WR star.  
The mass-luminosity relation of hydrogen-free WR stars implies a mass 
of $M_{\rm WR}\sim5\pm1.5M_{\odot}$.

From our data we favor an age of $\sim10$ Myr for the bulk of the 
Vela OB2 stars.  Evolutionary scenarios for $\zeta$~Pup and 
$\gamma^2$~Vel are discussed in the light of our results.

\end{abstract}
\end{document}