The Hubble Space Telescope key project on the Extragalactic Distance Scale. XX. The discovery of cepheids in the Virgo Cluster galaxy NGC 4548

We report on the discovery and properties of Cepheid variable stars in the barred spiral galaxy NGC 4548, which is a member of the Virgo cluster of ga laxies. This is one of the galaxies being observed as part of the Hubble: S pace Telescope (HST) Key Project on the Extragalactic Distance Scale, which aims to determine the Hubble constant to 10% accuracy. Our analysis is bas ed on observations made with the Wide Field and Planetary Camera 2 during 1 996 and 1997. We identify 24 probable Cepheids with periods between 16 and 38 days. They were observed over 13 epochs with the F555W filter and eight epochs with the F814W filter. The HST F555W and F814W data have been transf ormed to the Johnson V and Cousins I magnitude systems, respectively. Photo metry has principally been carried out using the DAOPHOT/ALLFRAME package. A comparison is made with parallel measurements using the DoPHOT package. A pparent period-luminosity relations for V and I have been constructed., Ass uming values of mu(0) = 18.50 +/- 0.10 mag and E(B-V) = 0.10 mag for the di stance modulus and reddening of the Large Magellanic Cloud, a true distance modulus of 31.01 +/- 0.28 mag is derived corresponding to a distance of 15 .9 +/- 2.0 Mpc. HST Cepheid distances of other spiral galaxies in the Virgo Cluster are discussed. Their good agreement with the new NGC 4548 distance strengthens the evidence that this galaxy lies within the Virgo cluster co re.