Direct measurement of the radius and density of the transiting exoplanet HD 189733B with the Chara array

We have measured the angular diameter of the transiting extrasolar planet host star HD 189733 using the CHARA optical/IR interferometric array. Combining our new angular diameter of 0.377 +/- 0.024 mas with the Hipparcos parallax leads to a linear radius for the host star of 0.779 +/- 0.052 R circle dot and a radius for the planet of 1.19 +/- 0.08 R-Jup. Adopting the mass of the planet as derived by its discoverers, we derive a mean density of the planet of 0.91 +/- 0.18 g cm(-3). This is the first determination of the diameter of an extrasolar planet through purely direct means.