Synthetic stellar spectra have been computed for Cepheids and supergiant F and G stars, using flux constant model atmospheres. These spectra have been used to compute theoretical uvby and UBV colours which may be compared with the observed colours. The supergiant α Per is used to establish the zero points of the theoretical colours. The colour comparison can be used to determine the temperatures, gravities and metal abundances of the stars as well as an estimate of the interstellar reddening. The reddening determinations are carried out by arguing that the temperature and gravity values obtained from U – B and B – V must match those obtained from c1 and b – y.

A more detailed account of this work will be given elsewhere.