The relation between the surface velocity of a pulsating star in the star's inertial frame and the apparent expansion velocity measured by a remote observer has been classically formulated in terms of a projection factor p. This factor depends on the relative limb darkening in continuum and spectral lines and may therefore be a function of composition. We have computed synthetic spectra to investigate the behaviour of p and, indeed, of the apparent line profile for radially pulsating stars over a range of temperature and gravity with hydrogen-rich and hydrogen-poor atmospheres. We have subsequently derived values of p suitable for the interpretation of velocity measurements of pulsating early type stars. Effective temperature has the most important influence on p: a change of 10 000 K, in T_eff introduces a change of 2-3 per cent with important consequences for measurements of stellar radii to an accuracy <1%.