  A campaign of 35 epochs of milli-arcsecond resolution VLBA observations of
the archetype colliding wind WR+O star binary system WR140 show the
wind-collision region (WCR) as a bow-shaped arc of emission that rotates as the
highly eccentric orbit progresses. The observations comprise 21 epochs from the
1993-2001 orbit, discussed by Dougherty et al. (2005), and 14 epochs from the
2001-2009 orbit, and span orbital phase 0.43 to 0.95. Assuming the WCR is
symmetric about the line-of-centres of the two stars and "points" at the WR
star, this rotation shows the O star moving from SE to E of the WR star between
these orbital phases. Using IR interferometry observations from IOTA that
resolve both stellar components at phase 0.297, in conjuction with orbital
parameters derived from radial velocity variations, the VLBA observations
constrain the inclination of the orbit plane as 120\degree \pm 4 \degree, the
longitude of the ascending node as 352\degree \pm 2 \degree, and the orbit
semimajor axis as 9.0 \pm 0.1 mas. This leads to a distance estimate to WR140
of 1.81 \pm 0.08 kpc. Further refinements of the orbit and distance await more
IR interferometric observations of the stellar components directly.

Solar and Stellar Astrophysics