  We present HST STIS observations of two occultations of the transiting
exoplanet HAT-P-1b. By measuring the planet to star flux ratio near opposition,
we constrain the geometric albedo of the planet, which is strongly linked to
its atmospheric temperature gradient. An advantage of HAT-P-1 as a target is
its binary companion ADS 16402 A, which provides an excellent photometric
reference, simplifying the usual steps in removing instrumental artifacts from
HST time-series photometry. We find that without this reference star, we would
need to detrend the lightcurve with the time of the exposures as well as the
first three powers of HST orbital phase, and this would introduce a strong bias
in the results for the albedo. However, with this reference star, we only need
to detrend the data with the time of the exposures to achieve the same
per-point scatter, therefore we can avoid most of the bias associated with
detrending. Our final result is a 2 sigma upper limit of 0.64 for the geometric
albedo of HAT-P-1b between 577 and 947 nm.
