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

Binary-driven stellar rotation evolution at the main-sequence turn-off in star clusters