a\ell \right)^{2} -
     \Delta_{r} 
                                        \left(\ell - a\sin^{2}\theta
     \right)^{2}\right]^{1/2}}\right\}.$$ The best insight into the $\ell = \mbox{const}$ configurations is given by properties of $W\left(r, \theta\right)$ in the equatorial plane ($\theta =
\pi/2$). The reality conditions of $W\left(r, \theta = \pi/2\right)$ imply $$\ell_{\rm ph-} < \ell < \ell_{\rm ph+},
\label{eq5}$$ where the functions $\ell_{\rm ph \pm} \left(r; a, y\right)$, given by $$\ell_{\rm ph\pm}(r;a,y) = a + \frac{r^2}{a\pm\sqrt{\Delta_r}},
                                                              \label{eq7}$$ determine the photon geodesic motion.[@Stu-Hle:2000:CLAQG:; @Stu-Sla:2004:PHYSR4:]

Condition for the local extrema of the potential $W\left(r, \theta =
\pi/2\right)$ is identical with the condition of vanishing of the pressure gradient $\left(\lpder{U_t}{r} = 0 =
\lpder{U_t}{\theta}\right)$. Since in the equatorial plane there is $\lpder{U_t}{\theta} = 0$, independently of $\ell = \mbox{const}$, the only relevant condition is $\lpder{U_t}{r} = 0$, which implies the relation $$\ell=\ell_{\rm K \pm}(r;a,y)$$ with $\ell_{\rm K \pm}$ being the angular momentum density of the geodetical Keplerian orbits $$\ell_{\rm K \pm} \left(r; a, y\right) \equiv \pm
  \frac{(r^{2}+a^{2})(1-yr^{3})^{1/2}\mp ar^{1/2}[2+yr(r^{2}+a^{2})]}%
       {r^{3/2}[1-y(r^{2}+a^{2})]-2r^{1/2}\pm a(1-yr^{3})^{1/2}}.
                                                               \label{eq12}$$ The closed equipotential surfaces, and surfaces with a cusp allowing the outflow of matter from the disc, are permitted in those parts of the functions $\ell_{\rm K \pm} \left(r; a, y\right)$ enabling the existence of stable circular geodesics corresponding to the centre of 