Uncertainties in the interstellar extinction curve and the Cepheid distance to M101

I revisit the Cepheid-distance determination to the nearby spiral galaxy M101 (Pinwheel Galaxy) of Shappee & Stanek, in light of several recent investigations questioning the shape of the interstellar extinction curve at lambda approximate to 8000 angstrom (i.e. the I band). I find that the relatively steep extinction ratio A(I)/E(V-I) = 1.1450 from Fitzpatrick & Massa is slightly favoured relative to A(I)/E(V-I) = 1.2899 from Fitzpatrick and significantly favoured relative the historically canonical value of A(I)/E(V-I) = 1.4695, from Cardelli et al. The steeper extinction curves, with lower values of A(I)/E(V-I), yield fits with reduced scatter, metallicity dependences to the dereddened Cepheid luminosities that are closer to values inferred in the Local Group, and that are less sensitive to the choice of reddening cut imposed in the sample selection. The increase in distance modulus to M101 when using the preferred extinction curve is Delta mu similar to 0.06 mag, resulting in the estimate of the distance modulus to M101 relative to the LMC is Delta(mu LMC) approximate to 10.72 +/- 0.03 (stat). The best-fitting metallicity dependence is dM(I)/d[O/H] approximate to (-0.38 +/- 0.14 (stat)) mag dex(-1).