Predicted Lyα equivalent width as a function of metallicity for constant star formation (left panel) and very young (≤1−2 Myr) bursts (right panel). Note the different vertical scales on the two plots. Thin lines show our “standard” predictions, thick lines the predicted W(Lyα) accounting to first order for departure from case B following Eq. (8) (assuming low density, i.e. , and neglecting the increase of the two-photon continuum), leading to an increase by up to a factor ~1.5−2.5 at low metallicities (Z ≲ Z_coll). Results for different IMFs are shown using the same colour codes as in Fig. 1 (cf. Table 1).