The p-factor values derived for each of the HST parallax Cepheids with the only constraint being that the IRSB distance should agree with the parallax distance. The full line shows the linear regression fit to the points. The line labelled “LMC and HST ZP” shows our adopted relation based on the constraints from the LMC Cepheids and forcing the distance zero point to agree with the HST parallax values. The stippled line delineates the theoretical relation from Nardetto et al. (2009) and the long dashed line the classical Hindsley & Bell (1986) relation. The horizontal line at p = 1.5 shows the physical limit above which the theoretical p-factor would indicate an unphysical limb-brightening.