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The late-type dwarf GJ436 is known to host a transiting Neptune-mass planet in a 2.6day orbit. We present results of our interferometric measurements to directly determine the stellar diameter (R ⊙ = 0.455 ± 0.018 R ⊙) and effective temperature (T EFF = 3416 ± 54 K). We combine our stellar parameters with literature time-series data, which allows us to calculate physical and orbital system parameters, including GJ436's stellar mass (M ⊙ = 0.507+0.071- 0.062 M ⊙), stellar density (ρ * = 5.37+0.30- 0.27 ρ ⊙), planetary radius (Rp = 0.369+0.015- 0.015 R Jupiter), and planetary mass (Mp = 0.078+0.007- 0.008 M Jupiter), implying a mean planetary density of ρp = 1.55+0.12- 0.10 ρJupiter. These values are generally in good agreement with previous literature estimates based on assumed stellar mass and photometric light curve fitting. Finally, we examine the expected phase curves of the hot Neptune GJ436b, based on various assumptions concerning the efficiency of energy redistribution in the planetary atmosphere, and find that it could be constrained with Spitzer monitoring observations. © 2012. The American Astronomical Society. All rights reserved..

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Astrophysical Journal