Plots showing the observed properties of star-forming regions, NGC 3621 is shown in the top two panels, and M83 is shown in the bottom two. Left: observed FUV luminosity against the Hα luminosity after dust corrections. Objects found in the inner disc (R≤R₂₅) are shown as drak grey (blue) diamonds, whilst objects found beyond R₂₅ are indicated by grey (red) dots. We have marked on the FUV and Hα luminosity for single O-type stars from Vacca et al. (1996) and Sternberg, Hoffmann & Pauldrach (2003). The solid black lines are evolutionary models from the starburst99 code, labelled for cluster masses of 10⁴, 10⁵ and 10⁶ M_⊙, grey dotted lines link clusters at common ages. Right: UV colour–magnitude diagrams for all detected objects. The colours represent the same distinction as the left-hand graphs. Black crosses mark the extended disc objects found to have associated Hα emission. Solid black lines show the same evolutionary models as before, masses are labelled from top to bottom as 10⁶, 10⁵, 10⁴ and 10³ M_⊙. In addition the solid black diamonds represent cluster ages from left to right of 10⁶, 10^6.5, 10⁷, 10^7.5, 10⁸ and 10^8.5 yr.