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 grou p of galaxies. NGC 3351 is one of 18 galaxies being observed as part o f the Hubble Space Telescope (HST) Key Project on;the Extragalactic Di stance Scale which aims to determine the Hubble constant to 10% accura cy. Our analysis is based on observations made with the Wide Field Pla netary Camera 2 during 1994 and early 1995. The Leo I group contains s everal bright galaxies of diverse types and is very suitable for linki ng together a number of secondary calibrators which can be employed at much greater distances than the Cepheid variables. We identify 49 pro bable Cepheids within NGC 3351 in the period range 10-43 days which ha ve 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. Photom etry has principally been;carried out using the DAOPHOT/ALLFRAME packa ge. Reference is made to parallel measurements being made with the DoP HOT package. Apparent period-luminosity functions for V and I have bee n 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 Magell anic Cloud. A true distance modulus of 30.01 +/- 0.19 mag is derived c orresponding 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 planetar y nebula luminosity function methods as calibrated by the Cepheids in M31.