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We report high-cadence time-series photometry of the recently-discoveredtransiting exoplanet system HD 17156, spanning the time of transit on UT 2007October 1, from three separate observatories. We present a joint analysis ofour photometry, previously published radial velocity measurements, and times oftransit center for 3 additional events. Adopting thespectroscopically-determined values and uncertainties for the stellar mass andradius, we estimate a planet radius of Rp = 1.01 +/- 0.09 RJup and aninclination of i = 86.5 +1.1 -0.7 degrees. We find a time of transit center ofTc = 2454374.8338 +/- 0.0020 HJD and an orbital period of P = 21.21691 +/-0.00071 days, and note that the 4 transits reported to date show no sign oftiming variations that would indicate the presence of a third body in thesystem. Our results do not preclude the existence of a secondary eclipse, butimply there is only a 9.2% chance for this to be present, and an even lowerprobability (6.9%) that the secondary eclipse would be a non-grazing event. Dueto its eccentric orbit and long period, HD 17156b is a fascinating object forthe study of the dynamics of exoplanet atmospheres. To aid such future studies,we present theoretical light curves for the variable infrared emission from thevisible hemisphere of the planet throughout its orbit.

Parameters and Predictions for the Long-Period Transiting Planet HD 17156b