Comparison of colour-period distribution of Group X with age-benchmark open clusters. Top panel: distribution of stellar rotation periods of Group X with the Pleiades and Praesepe distributions overplotted. Bottom panel: distribution of the rotation periods of slow rotators of Group X versus B − V colour, with the polynomial fits to median rotation periods of slow rotators in the Praesepe (red line) and Pleiades (blue line) clusters and, as an example, the gyro-sequence corresponding to an age of 300 Myr (green line), according to the Mamajek & Hillenbrand (2008) coefficients, overplotted.