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A. G. Riess (STScI), High-z Supernova Team
We have measured the rest-frame B,V, and I-band light curves of a high-redshift type Ia supernova (SN Ia), SN 1999Q (z=0.46), using HST and ground-based near-IR detectors. A goal of this study is the measurement of the color excess, EB-I, which is a sensitive indicator of interstellar or intergalactic dust which could affect recent cosmological measurements from high-redshift SNe Ia. Our observations are moderately inconsistent with enough reddening by either Galactic-type or gray (size > 0.1 \ \mum) dust to reconcile the observations of high-redshift SNe Ia with \Omega\Lambda=0 and q0 \geq 0. The observed I-band light curve shows the secondary maximum typical of nearby SNe Ia of normal luminosity, providing no obvious indication of evolution as a function of redshift out to z ~0.5. An expanded set of similar observations could yield the strongest constraints on challenges to indications from SNe Ia that the expansion of the Universe is accelerating.