Multiband Cepheid PL relations from OGLE III measurements and the impact on Hubble's constant

The Cepheid PL relation is of fundamental importance in estimating Hubble's constant. The Key Project has estimated H0 with an accuracy of 10{\%}. Recent work has suggested that a CMB independent estimate of H0 accurate to less than 5{\%} can constrain the equation of state for Dark Energy. In this work, we show that one of the fundamental assumptions underlying the PL relation, that it is linear, is not supported by recent observations. We show that this can affect estimates of H0 by as much as 1-2{\%}: small but perhaps important in the context of reducing errors on H0 to less than 5{\%}.