Chaotic zone (in dark gray) as a function of the binary eccentricity, for binary components of same masses. The lighter part designates a critical zone, where some test particles can survive. The red lines represent the lower and upper critical orbit parabolic fits found by Dvorak (1986) in its study of circumbinary planet stability. The 1:6 commensurability is the strongest outside the chaotic zone (see Sect. 2.2) for e_B ≥ 0.4.