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We present direct radii measurements of the well-known transiting exoplanethost stars HD 189733 and HD 209458 using the CHARA Array interferometer. Wefind the limb-darkened angular diameters to be theta_LD = 0.3848 +/- 0.0055 and0.2254 +/- 0.0072 milliarcsec for HD 189733 and HD 209458, respectively. HD189733 and HD 209458 are currently the only two transiting exoplanet systemswhere detection of the respective planetary companion's orbital motion fromhigh resolution spectroscopy has revealed absolute masses for both star andplanet. We use our new measurements together with the orbital information fromradial velocity and photometric time series data, Hipparcos distances, andnewly measured bolometric fluxes to determine the stellar effectivetemperatures (T_eff = 4875 +/- 43, 6093 +/- 103 K), stellar linear radii (R_* =0.805 +/- 0.016, 1.203 +/- 0.061 R_sun), mean stellar densities (rho_* = 1.62+/- 0.11, 0.58 +/- 0.14 rho_sun), planetary radii (R_p = 1.216 +/- 0.024, 1.451+/- 0.074 R_Jup), and mean planetary densities (rho_p = 0.605 +/- 0.029, 0.196+/- 0.033 rho_Jup) for HD 189733 b and HD 209458 b, respectively. The stellarparameters for HD 209458, a F9 dwarf, are consistent with indirect estimatesderived from spectroscopic and evolutionary modeling. However, we find thatmodels are unable to reproduce the observational results for the K2 dwarf, HD189733. We show that, for stellar evolutionary models to match the observedstellar properties of HD 189733, adjustments lowering the solar-calibratedmixing length parameter from 1.83 to 1.34 need to be employed.

monthly notices of the royal astronomical society

Stellar Diameters and Temperatures VI. High angular resolution  measurements of the transiting exoplanet host stars HD 189733 and HD 209458  and implications for models of cool dwarfs