Comparison between burst population synthesis models including VMSs up to 225 M_⊙ to the observed UV spectrum of cluster A in II Zw 40 (black line). All models are for Z = 0.2 Z_⊙ that is close to the metallicity of the cluster. Ages range from 0 to 2.5 Myr and are colour-coded as indicated in the top panel. The top (bottom) panel corresponds to models without (with) metallicity scaling of the VMS mass-loss rates. The vertical black lines indicate the Galactic nebular C IV absorption.