  The impact of stellar rotation on the morphology of star cluster
colour-magnitude diagrams is widely acknowledged. However, the physics driving
the distribution of the equatorial rotation velocities of main-sequence
turn-off (MSTO) stars is as yet poorly understood. Using Gaia Data Release 2
photometry and new Southern African Large Telescope medium-resolution
spectroscopy, we analyse the intermediate-age ($\sim1\,$Gyr-old) Galactic open
clusters NGC 3960, NGC 6134 and IC 4756 and develop a novel method to derive
their stellar rotation distributions based on SYCLIST stellar rotation models.
Combined with literature data for the open clusters NGC 5822 and NGC 2818, we
find a tight correlation between the number ratio of slow rotators and the
clusters' binary fractions. The blue-main-sequence stars in at least two of our
clusters are more centrally concentrated than their red-main-sequence
counterparts. The origin of the equatorial stellar rotation distribution and
its evolution remains as yet unidentified. However, the observed correlation in
our open cluster sample suggests a binary-driven formation mechanism.
