The effect of metallicity on Cepheid magnitudes and the distance to M33

We present the results from a multi-epoch survey of two regions of M33 using the 3.5-m Wisconsin-Indiana-Yale-NOAO (WIYN) telescope. The inner field is located close to the centre of the galaxy, with the outer region situated about 5.1 kpc away in the southern spiral arm, allowing us to sample a large metallicity range. We have data for 167 fundamental mode Cepheids in the two regions. The reddening-free Wesenheit magnitude W-vi period-luminosity (PL) relations were used to establish the distance modulus of each region, with mu(inner) = 24.37 +/- 0.02 mag and mu(outer) = 24.54 +/- 0.03 mag. The apparent discrepancy between these two results can be explained by the significant metallicity gradient of the galaxy. We determine a value for the metallicity parameter of the PL relation gamma = delta(m - M)(0)/delta log Z = -0.29 +/- 0.11 mag dex(-1) consistent with previous measurements. This leads to a metallicity corrected distance modulus to M33 of mu(gamma) = 24.53 +/- 0.11 mag.