The Hubble Space Telescope Extragalactic Distance Scale Key Project .7. The discovery of Cepheids in the Leo I group galaxy NGC 3351

We report of the discovery and properties of Cepheid variable stars in the barred spiral galaxy NGC 3351 which is a member of the Leo I group of galaxies. NGC 3351 is one of 18 galaxies being observed as part of the Hubble Space Telescope (HST) Key Project on;the Extragalactic Distance Scale which aims to determine the Hubble constant to 10% accuracy. Our analysis is based on observations made with the Wide Field Planetary Camera 2 during 1994 and early 1995. The Leo I group contains several bright galaxies of diverse types and is very suitable for linking together a number of secondary calibrators which can be employed at much greater distances than the Cepheid variables. We identify 49 probable Cepheids within NGC 3351 in the period range 10-43 days which have been observed at 12 epochs with the F555W filter and 4 epochs using the F814W filter. The HST F555W and F814W data have been transformed to the Johnson V and Cousins I magnitude systems, respectively. Photometry has principally been;carried out using the DAOPHOT/ALLFRAME package. Reference is made to parallel measurements being made with the DoPHOT package. Apparent period-luminosity functions for V and I have been constructed assuming values of mu(0) = 18.50 +/- 0.10 mag and E(B-V) = 0.10 mag for the distance modulus and reddening of the Large Magellanic Cloud. A true distance modulus of 30.01 +/- 0.19 mag is derived corresponding to a distance of 10.05 +/- 0.88 Mpc with a reddening E(V-I) = 0.15 mag. A comparison is made with distances estimated for other galaxies in the Leo I group using various distance indicators. There is good agreement with the surface brightness fluctuation and planetary nebula luminosity function methods as calibrated by the Cepheids in M31.