Left: predicted UV slope β₁₅₀₀ and for all IMFs and metallicities. The values are shown for very young (ZAMS) populations, which correspond to the bluest possible slopes (i.e. minimal β values). The upper set of lines shows the UV slopes of the total spectrum (stellar + nebular continuum), the lower lines using the pure stellar spectrum. Results for different IMFs are shown using the same colour codes as in Fig. 1 (cf. Table 1). Right: same as the left panel, for β₂₀₀₀. Note the difference between β₁₅₀₀ and β₂₀₀₀, indicative of the deviation of the true SED from a simple power law.