  The distance modulus of a galaxy determined from Cepheids differs from its
distance modulus determined from Tully-Fisher by an amount that is proportional
to the galaxy's line width, W: Delta mu ~ (-1.73\pm 0.46)log(W). While a
miscalibration of the slope of Tully-Fisher could in principle produce this
effect, we argue that such a miscalibration is very unlikely. The other
possible explanation is that the inferred Cepheid luminosity is correlated with
the rotation speed v (and hence W) of its parent galaxy: L ~ v^{0.7}. This
proportionality is superposed on the well-established relation between period
and luminosity.) Such a dependence would be expected if Cepheid luminosity is
correlated with metallicity, since galaxies with deeper potential wells tend to
retain more of their metals. It would induce a discrepancy of 0.83 mag between
H_0 determinations that are calibrated using M31 (log W=2.71) and the H_0
determination from SNIa, which is calibrated in galaxies with mean line width
<log W>=2.23. A discrepancy of 0.83 mag corresponds to the difference between
80 km/s/Mpc and 55 km/s/Mpc, very similar to the actual discrepancy reported in
the current literature.
