The p-factor predicted from plane-parallel and spherically-symmetric model atmospheres as a function of pulsation period. Projection factors computed from plane-parallel models are shown for the BVRIH and K-bands as blue dashed lines, while projection factors from spherically-symmetric models are shown as red solid lines. For reference the period–projection factor relations are shown from Hindsley & Bell (1986) (magenta dotted line), Nardetto et al. (2007), their Eq. (6) (orange dashed-double dot line), Nardetto et al. (2009) (pale blue dot-dashed line) and Storm et al. (2011b), (black double-dashed line). The projection factors for Galactic Cepheids determined by Storm et al. (2011a) are also included (black dots).