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Young and intermediate-age star clusters of both Magellanic Clouds exhibitcomplex color-magnitude diagrams. In addition to the extended main-sequenceturn-offs (eMSTOs), commonly observed in star clusters younger than ~2 Gyr, theclusters younger than ~800 Myr exhibit split main sequences (MSs). Thesecomprise a blue MS, composed of stars with low-rotation rates, and a red MS,which hosts fast-rotating stars. While it is widely accepted that stellarpopulations with different rotation rates are responsible for the eMSTOs andsplit MSs, their formation and evolution are still debated. A recentinvestigation of the ~1.7 Gyr old cluster NGC1783 detected a group of eMSTOstars extremely dim in UV bands. Here, we use multi-band Hubble Space Telescopephotometry to investigate five star clusters younger than ~200 Myr, includingNGC1805, NGC1818, NGC1850, and NGC2164 in the Large Magellanic Cloud, and theSmall-Magellanic Cloud cluster NGC330. We discover a group of bright MS starsin each cluster that are significantly dim in the F225W and F275W bands,similar to what is observed in NGC1783. Our result suggests that UV-dim starsare common in young clusters. The evidence that most of them populate the blueMS indicates that they are slow rotators. As a byproduct, we show that the starclusters NGC1850 and BHRT5b exhibit different proper motions, thuscorroborating the evidence that they are not gravitationally bound.

Hubble Space Telescope survey of Magellanic Cloud star clusters. UV-dim stars in young clusters