  We present a detailed study of stellar rotation in the massive 1.5 Gyr old
cluster NGC 1846 in the Large Magellanic Cloud. Similar to other clusters at
this age, NGC 1846 shows an extended main sequence turn-off (eMSTO), and
previous photometric studies have suggested it could be bimodal. In this study,
we use MUSE integral-field spectroscopy to measure the projected rotational
velocities (vsini) of around 1400 stars across the eMSTO and along the upper
main sequence of NGC 1846. We measure vsini values up to ~250 km/s and find a
clear relation between the vsini of a star and its location across the eMSTO.
Closer inspection of the distribution of rotation rates reveals evidence for a
bimodal distribution, with the fast rotators centred around vsini = 140 km/s
and the slow rotators centred around vsini = 60 km/s. We further observe a lack
of fast rotating stars along the photometric binary sequence of NGC 1846,
confirming results from the field that suggest that tidal interactions in
binary systems can spin down stars. However, we do not detect a significant
difference in the binary fractions of the fast and slowly rotating
sub-populations. Finally, we report on the serendipitous discovery of a
planetary nebula associated with NGC 1846.
