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Repeated imaging observations have been made of NGC 3982 with the HubbleSpace Telescope between March and May 2000, over an interval of 53 days. Imageswere obtained on 12 epochs in the F555W band and on five epochs in the F814Wband. The galaxy hosted the type Ia supernova SN1998aq.  A total of 26 Cepheid candidates were identified, with periods ranging from10 to 45 days, using photometry with the DoPHOT program. The de-reddeneddistance to NGC 3982 is estimated from these data using various criteria tomaximize signal to noise and reliability: the values lie between 31.71 and31.82, with uncertainties in the mean of typically +/- 0.14 mag for each case.A parallel analysis using photometry with HSTphot discovered 13 variables,yielding a distance modulus of 31.85 +/- 0.16. The final adopted modulus is(M-m)_0 = 31.72 +/- 0.14 (22 +/- 1.5 Mpc).  Photometry of 1998aq that is available in the literature is used incombination with the derived distance to NGC 3982 to obtain the peak absolutemagnitude of this supernova. The lower limit (no extinction within the hostgalaxy) for M_V is -19.47 +/- 0.15 mag. Corrections for decline rate andintrinsic color to carry these to the reduced system of Parodi et al. (2000)have been performed. The derived luminosities at hand are fully consistent withthe mean of the 8 normal SNe Ia previously calibrated with Cepheids. Togetherthey yield H_0 ~ 60 +/- 2 (internal) km s-1 Mpc-1 based on an assumed LMCdistance modulus of 18.50. We point out that correcting some of the systematicerrors and including uncertainty estimates due to them leads to H_0 = 58.7 +/-6.3 (internal) km s-1 Mpc-1.Comment: 53 pages, 8 figure

Cepheid Calibration of the Peak Brightness of Type Ia Supernovae. XI. SN 1998aq in NGC 3982