2002ApJ...565..959L

We have determined the distance to M33 using single-epoch I-band observations of Cepheids based on Hubble Space Telescope (HST)/Wide Field Planetary Camera 2 (WFPC2) images of five fields in M33. Combining the HST I-band photometry and the periods determined from the ground-based observations (DIRECT) for 21 Cepheids with logP>0.8 in the sample of 32 Cepheids, we derive a distance modulus of (m-M)₀=24.52±0.14(random)±0.13(systematic) for an adopted total reddening of M33, E(B-V)=0.20±0.04 [E(V-I)=0.27±0.05] given by Freedman et al.; the reddening to the LMC, E(B-V)=0.10; and the distance to the LMC, (m-M)₀=18.50. If the total reddening to M33 of E(B-V)=0.10±0.09 given by Freedman, Wilson, and Madore is used, the Cepheid distance modulus based on the I-band photometry will be increased by 0.20. Metallicity effect on the Cepheid distance to M33 is estimated to be small, δ(m-M)_Z=0.01-0.06, which leads to (m-M)₀=24.53-24.58 after this metallicity effect correction. Using the Wesenheit W_I, an extinction-free parameter, we derive a similar value, (m-M)₀=24.52±0.15(random) ±0.11(systematic). These results are in reasonable agreement with those based on the ground-based multiepoch BVRI observations of brighter Cepheids in M33 and are ~0.3 smaller than those based on the tip of the red giant branch and the red clump. It is needed to estimate better the reddening to Cepheids in M33.