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 ∼139 pc and projected separation of ∼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 0.53 M J and radius ∼1.36 R J traveling in an orbit with semimajor axis 0.055 AU and inclination about 85. • 9, thus transiting the star at impact parameter 0.74 of the stellar radius. Based on a data set spanning three years, ephemerides for the transit center are: T C = 2453984.397 + N tr * 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.

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ABSTRACTwhich falls in the planetary-mass regime. This is the first discovery of a planetary companion orbiting a G-type giant star.