of the external spacetime. All characteristic cases of its behaviour are presented for both ECS and non-extreme compact stars.[]{data-label="FIGVdiff"}](Fig2c.pdf "fig:"){width="\hsize"}

![The effective potential $V_{\mathrm{eff}}$ of both internal (dashed lines) and external spacetimes (full lines) given for the black-hole type ($b < 1$) and naked-singularity ($b > 1$) type of the external spacetime. All characteristic cases of its behaviour are presented for both ECS and non-extreme compact stars.[]{data-label="FIGVdiff"}](Fig2b.pdf "fig:"){width="\hsize"} ![The effective potential $V_{\mathrm{eff}}$ of both internal (dashed lines) and external spacetimes (full lines) given for the black-hole type ($b < 1$) and naked-singularity ($b > 1$) type of the external spacetime. All characteristic cases of its behaviour are presented for both ECS and non-extreme compact stars.[]{data-label="FIGVdiff"}](Fig2d.pdf "fig:"){width="\hsize"}

![The effective potential $V_{\mathrm{eff}}$ of both internal (dashed lines) and external spacetimes (full lines) given for the black-hole type ($b < 1$) and naked-singularity ($b > 1$) type of the external spacetime. All characteristic cases of its behaviour are presented for both ECS and non-extreme compact stars.[]{data-label="FIGVdiff"}](Fig2e.pdf){width=".49\hsize"}

Circular null geodesics, located at $r_\mathrm{c(i)}$ and $r_\mathrm{c(e)}$, respectively, are given by the local extrema of the effective potential ($\partial {V_{\mathrm{eff}}}/\partial {r} = 0$). In the internal spacetime we have to solve a nontrivial equation $r$: $$\begin{aligned}
      