Fig. 2. Synthetic UV spectra showing the spectral evolution on the 20 track. Plotted is the logarithm of the emergent luminosity. Approximate spectral types from Table 1 are given. Starting with the second model, each spectrum has been shifted downwards by 0.7 dex with respect to the previous one, in order to allow a good comparison. The marks on the top and bottom indicate the location of the CNO and Si lines taken from the lists of Bruhweiler et al.  (1981) and Dean & Bruhweiler (1985). The strongest CNO, and Si features are labeled