  Wolf-Rayet stars represent one of the final stages of massive stellar
evolution. Relatively little is known about this short-lived phase and we
currently lack reliable mass, distance, and binarity determinations for a
representative sample. Here we report the first visual orbit for WR
140(=HD193793), a WC7+O5 binary system known for its periodic dust production
episodes triggered by intense colliding winds near periastron passage. The IOTA
and CHARA interferometers resolved the pair of stars in each year from
2003--2009, covering most of the highly-eccentric, 7.9 year orbit. Combining
our results with the recent improved double-line spectroscopic orbit of Fahed
et al. (2011), we find the WR 140 system is located at a distance of 1.67 +/-
0.03 kpc, composed of a WR star with M_WR = 14.9 +/- 0.5 Msun and an O star
with M_O = 35.9 +/- 1.3 Msun. Our precision orbit yields key parameters with
uncertainties times 6 smaller than previous work and paves the way for detailed
modeling of the system. Our newly measured flux ratios at the near-infrared H
and Ks bands allow an SED decomposition and analysis of the component
evolutionary states.
