the related local extrema of the effective potential are given by equation (\[EQph\]) and their properties were discussed in the previous section.

When we take into account the (non-)existence of the local extrema of the internal/external part of the effective potential $V_\mathrm{eff}$, we obtain the whole region of existence of ECS. The function $R{}_\mathrm{max}^\mathrm{ECS}(b)$ is determined by the relation $$R{}_\mathrm{max}^{\mathrm{ECS}} =\left\{
        \begin{array}{lll}
          r_\mathrm{ph} \equiv \frac{1}{2} \left(3 + \sqrt{9-8 b}\right) & \quad\mbox{for} & b \leq 0\\
          R_\mathrm{I+} \equiv \frac{3}{2} + \sqrt{\frac{9}{4} - \frac{5}{3}b}  & \quad\mbox{for} & 0 < b < 27/20
        \end{array}
      \right.,$$ while for $R{}_\mathrm{min}^\mathrm{ECS}(b)$ we have $$R{}_\mathrm{min}^{\mathrm{ECS}} = \left\{
        \begin{array}{lll}
          R_\mathrm{min} & \quad\mbox{for} & b \leq 1\\
          R_\mathrm{I-} \equiv \frac{3}{2} - \sqrt{\frac{9}{4} - \frac{5}{3}b} & \quad\mbox{for}& 1 < b < 27/20
        \end{array}
      \right..$$ The region of ECS in the parameter space is represented in Figure \[FIGzone\].

Typical behaviour of the effective potential of the null-geodetical motion $V{}_{\mathrm{eff}}$ is demonstrated in Figure \[FIGVdiff\] for appropriatelly chosen values of the tidal charge $b$. Here, and henceforth, we express the radii in units of the gravitational radius $r_g = G\mathcal{M}$. The selection of the tidal charge values used in Figure \[FIGVdiff\] demonstrates the full classification of the behavior of the effective potential 