The Hubble Space Telescope Extragalactic Distance Scale Key Project. X. The Cepheid distance to NGC 7331

The distance to NGC 7331 has been derived from Cepheid variables observed with the Hubble Space Telescope (HST) WFPC2, as part of the Extragalactic Distance Scale Key Project. Multiepoch exposures in F555W (similar to V) and F814W (similar to I), with photometry derived independently from DoPHOT and DAOPHOT/ALLFRAME programs, were used to detect a total of 13 reliable Cepheids, with periods between 11 and 42 days. The relative distance moduli between NGC 7331 and the LMC, derived from the V and I magnitudes, imply an extinction to NGC 7331 of A(v) = 0.47 +/- 0.15 mag and an extinction-corrected distance modulus to NGC 7331 of 30.89 +/- 0.14 (random), equivalent to a distance of 15.1(-0.9)(+1.0) Mpc. There are additional systematic uncertainties in the distance modulus of +/-0.12 mag resulting from the calibration of the Cepheid period-luminosity relation and a systematic offset of +0.05 +/- 0.04 mag if the metallicity correction inferred from the M101 results of Kennicutt et al. are applied.