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We present measurements of 68 classical Cepheids with periods from 10 to 78days observed in the near-infrared by the PHAT Program using the Wide FieldCamera 3 (WFC3) on the Hubble Space Telescope (HST). The combination of HST'sresolution and the use of near-infrared measurements provides a dramaticreduction in the dispersion of the Period--Luminosity relation over the presentoptical, ground-based data. Even using random-phase magnitudes we measure adispersion of just 0.17 mag, implying a dispersion of just 0.12 mag for meanmagnitudes. The error in the mean for this relation is 1% in distance. Combinedwith similar observations of Cepheids in other hosts and independent distancedeterminations, we measure a distance to M31 of mu_0=24.42 +/- 0.05(statistical) +/- 0.03 (systematic), 765 +/- 28 kpc, in good agreement withpast measurements though with a better, 3% precision here. The result is alsoin good agreement with independent distance determinations from two detachedeclipsing binaries allowing for an independent calibration of the Cepheidluminosities and a determination of the Hubble constant.

Cepheid Period-Luminosity Relations in the Near-Infrared and the Distance to M31 from the Hubble Space Telescope Wide Field Camera 3