  The importance of stellar rotation in setting the observed properties of
young star clusters has become clearer over the past decade, with rotation
being identified as the main cause of the observed extended main sequence
turn-off (eMSTO) phenomenon and split main-sequences. Additionally, young star
clusters are observed to host large fractions of rapidly rotating Be stars,
many of which are seen nearly equator-on through decretion disks that cause
self-extinction (the so called "shell stars"). Recently, a new phenomenon has
been reported in the $\sim1.5$ Gyr star cluster NGC 1783, where a fraction of
the main sequence turn-off stars appears abnormally dim in the UV. We
investigate the origin of these "UV-dim" stars by comparing the UV
colour-magnitude diagrams of NGC 1850 ($\sim100$ Myr), NGC 1783 ($\sim1.5$
Gyr), NGC 1978 ($\sim2$ Gyr) and NGC 2121 ($\sim2.5$ Gyr), massive star
clusters in the Large Magellanic Cloud. While the younger clusters show a
non-negligible fraction of UV-dim stars, we find a significant drop of such
stars in the two older clusters. This is remarkable as clusters older than
$\sim$2 Gyr do not have an eMSTO, thus a large populations of rapidly rotating
stars, because their main sequence turn-off stars are low enough in mass to
slow down due to magnetic braking. We conclude that the UV-dim stars are likely
rapidly rotating stars with decretion disks seen nearly equator-on (i.e., are
shell stars) and discuss future observations that can confirm or refute our
hypothesis.
