  The extent of the continuous zone of chaotic orbits of a small-mass tertiary
around a system of two gravitationally bound primaries (a double star, a double
black hole, a binary asteroid, etc.) is estimated analytically, in function of
the tertiary's orbital eccentricity. The separatrix map theory is used to
demonstrate that the central continuous chaos zone emerges (above a threshold
in the primaries mass ratio) due to overlapping of the orbital resonances
corresponding to the integer ratios p:1 between the tertiary and the central
binary periods. In this zone, the unlimited chaotic orbital diffusion of the
tertiary takes place, up to its ejection from the system. The primaries mass
ratio, above which such a chaotic zone is universally present at all initial
eccentricities of the tertiary, is estimated. The diversity of the observed
orbital configurations of biplanetary and circumbinary exosystems is shown to
be in accord with the existence of the primaries mass parameter threshold.
