We report on the discovery of Cepheids in the field spiral galaxy NGC 3621 , based on observations made with the Wide Field and Planetary Camera 2 on board the Hubble Space Telescope ( HST ) .
NGC 3621 is one of 18 galaxies observed as a part of The HST Key Project on the Extragalactic Distance Scale , which aims to measure the Hubble constant to 10 % accuracy .
Sixty-nine Cepheids with periods in the range 9–60 days were observed over 12 epochs using the F555W filter , and 4 epochs using the F814W filter .
The HST F555W and F814W data were transformed to the Johnson V and Kron-Cousins I magnitude systems , respectively .
Photometry was performed using two independent packages , DAOPHOT II/ALLFRAME and DoPHOT . Period-luminosity relations in the V and I bands were constructed using 36 fairly isolated Cepheids present in our set of 69 variables .
Extinction-corrected distance moduli relative to the LMC of 10.63 \pm 0.09 mag and 10.56 \pm 0.10 mag were obtained using the ALLFRAME and DoPHOT data , respectively .
True distance moduli of 29.13 \pm 0.18 mag and 29.06 \pm 0.18 mag , corresponding to distances of 6.3 \pm 0.7 Mpc and 6.1 \pm 0.7 Mpc , were obtained by assuming values of \mu _ { 0 } = 18.50 \pm 0.10 mag and E ( V - I ) = 0.13 mag for the distance modulus and reddening of the LMC , respectively .