CMD of M 55 zoomed in the BSS region, with the measured BSSs highlighted as large colored circles. The slowly rotating BSSs are marked with small blue circles. The FR-BSSs are plotted as increasingly larger circles for different colors for increasing rotational velocity (in units of km s⁻¹ ): orange color for 40 ≤ υ sin(i) < 80, red color for 80 ≤ υ sin(i) < 180, and violet for υ sin(i) ≥ 180 km s⁻¹. All the BSSs identified as variable stars in the Kaluzny et al. (2010) catalog are highlighted with large open circles. The FR-BSS classified as contact binary is marked with a large black cross.