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We refine the physical parameters of the transiting hot Jupiter planet XO-1band its stellar host XO-1 using HST NICMOS observations. XO-1b has a radiusRp=1.21+/-0.03 RJup, and XO-1 has a radius Rs=0.94+/-0.02 RSun, where theuncertainty in the mass of XO-1 dominates the uncertainty of Rp and Rs. Thereare no significant differences in the XO-1 system properties between thesebroad-band NIR observations and previous determinations based upon ground-basedoptical observations. We measure two transit timings from these observationswith 9 s and 15 s precision. As a residual to a linear ephemeris model, thereis a 2.0 sigma timing difference between the two HST visits that are separatedby 3 transit events (11.8 days). These two transit timings and additionaltimings from the literature are sufficient to rule out the presence of an Earthmass planet orbiting in 2:1 mean motion resonance coplanar with XO-1b. Weidentify and correct for poorly understood gain-like variations present inNICMOS time series data. This correction reduces the effective noise in timeseries photometry by a factor of two, for the case of XO-1.

NICMOS Observations of the Transiting Hot Jupiter XO-1b