  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 AB. 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_jup and radius ~1.36 R_jup traveling in an orbit with
semimajor axis 0.055 AU and inclination about 85.9 deg, 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.
