HAT-P-1b:: A large-radius, low-density exoplanet transiting one member of a stellar binary

Using small automated telescopes in Arizona and Hawaii, the HATNet project has detected an object transiting one member of the double star system ADS 16402. This system is a pair of G0 main-sequence stars with age about 3 Gyr at a distance of similar to 139 pc and projected separation of similar to 1550 AU. The transit signal has a period of 4.46529 days and depth of 0.015 mag. From follow-up photometry and spectroscopy, we find that the object is a "hot Jupiter'' planet with mass about 0.53M(J) and radius similar to 1.36R(J) traveling in an orbit with semimajor axis 0.055 AU and inclination about 85.9 degrees, thus transiting the star at impact parameter 0.74 of the stellar radius. Based on a data set spanning 3 yr, ephemerides for the transit center are T-C = 2453984.397 + Nu 4.46529. The planet, designated HAT-P-1b, appears to be at least as large in radius, and smaller in mean density, than any previously known planet.