Uncertainty in the metal abundance dependence of the Cepheid variable period-luminosity ( PL ) relation remains one of the outstanding sources of systematic error in the extragalactic distance scale and the Hubble constant .
To test for such a metallicity dependence , we have used the Wide Field Planetary Camera 2 ( WFPC2 ) on the Hubble Space Telescope ( HST ) to observe Cepheids in two fields in the nearby spiral galaxy M101 , which span a range in oxygen abundance of 0.7 \pm 0.15 dex .
A differential analysis of the PL relations in V and I in the two fields yields a marginally significant change in the inferred distance modulus on metal abundance , with \delta ( m - M ) _ { 0 } / \delta [ O / H ] = -0.24 \pm 0.16 mag dex ^ { -1 } .
The trend is in the theoretically predicted sense that metal-rich Cepheids appear brighter and closer than metal-poor stars .
External comparisions of Cepheid distances with those derived from three other distance indicators , in particular the tip of the red giant branch method , further constrain the magnitude of any Z -dependence of the PL relation at V and I .
The overall effects of any metallicity dependence on the distance scale derived with HST will be of the order of a few percent or less for most applications , though distances to individual galaxies at the extremes of the metal abundance range may be affected at the 10 % level .