(\[e58\]), determines marginally stable orbits of the plus-family, while the function $a^2_{\mathrm{ms(2)}}$, corresponding to the $(-)$ sign in Eq. (\[e58\]), is relevant for both the plus-family and minus-family orbits. A detailed analysis shows that the critical value of the cosmological parameter for the existence of the stable (plus-family) orbits is given by $$y_{\mathrm{crit(ms+)}} = \frac{100}{(5 + 2\sqrt{10})^{3}}\doteq 0.06886.
                                                        \label{ee1}$$ No stable circular orbits (of any family) exist for $y>y_{\mathrm{crit(ms+)}}$. The critical value of $y$ for the existence of the minus-family stable circular orbits is given by $$y_{\mathrm{crit(ms-)}} = \frac{12}{15^{4}}.             \label{ee3}$$ It coincides with the limit on the existence of the stable circular orbits in the SdS spacetimes.[@Stu-Hle:1999:PHYSR4:] In the parameter space $y$–$a^{2}$, separation of the KdS spacetimes according to the existence of stable circular orbits is given in Fig. \[f1\].

Behaviour of the effective potential (\[e23\]) enables us to introduce the notion of marginally bound orbits, i.e., unstable circular orbits where a small radial perturbation causes infall of a particle from the orbit to the centre, or its escape to the cosmological horizon. For some special value of the axial parameter $X = L - aE$, denoted as $X_{\mathrm{mb}}$, the effective potential has two local maxima related by $$E_{(+)}(r_{\mathrm{mb(i)}};X_{\mathrm{mb}},a,y)
     =E_{(+)}(r_{\mathrm{mb(o)}};X_{\mathrm{mb}},a,y),$$ 