Projection effects in the spectra of early-type pulsating stars

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 ob server has been classically formulated in terms of a projection factor p. T his factor depends on the relative limb darkening in continuum and spectral lines and may therefore be a function of composition. We have computed syn thetic spectra to investigate the behaviour of p and, indeed, of the appare nt line profile for radially pulsating stars over a range of temperature an d gravity with hydrogen-rich and hydrogen-poor atmospheres. We have subsequ ently derived values of p suitable for the interpretation of velocity measu rements of pulsating early type stars. Effective temperature has the most i mportant 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 rad ii to an accuracy <1%.