Area on the CMD for Stock 2, NGC 1960 and FSR 866 open cluster based on stars with the membership probability P > 0.95, P > 0.95 and P > 0.985, respectively, where binary fraction were investigated. The dashed border indicates the boundaries of the study area. For more details on its determination see Section 4. Gray dots are stars beyond the area. The green part is the area below the line q = 0.6, while the red one is the area above the line q = 0.6, which is considered to contain binary MS stars. Colors are preliminarily dereddened.