the extreme hole state ($a<4\sqrt{2}/(3\sqrt{3}),\
y=0$), even stable plus-family orbits with $E < 0$ can exist,[@Stu-Sla:2004:PHYSR4:] see Fig. \[f1\].

Angular velocity $\Omega=\d\phi/\d t$ of a thin, Keplerian accretion disc is given by $$\label{e5}
     \Omega_{\rm K\pm} = \pm\frac{1}{r^{3/2}/(1-yr^3)^{1/2} \pm a}.$$ Matter in the thin disc spirals from the outer marginally stable orbit through the sequence of stable circular orbits down to the inner marginally stable orbit losing energy and angular momentum due to the viscosity. The necessary conditions for such a differential rotation $$\label{E6}
     \frac{\d\Omega_{\rm K+}}{\d r}<0 \quad \frac{\d L_+}{\d r}\geq 0 
     \qquad\mbox{or}\qquad 
     \frac{\d\Omega_{\rm K-}}{\d r}>0 \quad \frac{\d L_-}{\d r}\leq 0,$$ are fulfilled by the relations (\[e40\]) and (\[e5\]). The efficiency of accretion, i.e., the efficiency of conversion of rest mass into heat energy of any element of matter transversing the discs from their outer edge located on the outer marginally stable orbit to their inner edge located on the inner marginally stable orbit is given by $$\eta \equiv E_{\mathrm{ms(o)}}-E_{\mathrm{ms(i)}}.$$ For Keplerian discs co-rotating extreme KdS black holes, the accretion efficiency reaches maximum value of $\eta \sim 0.43$ for the pure Kerr case ($y=0$) and tends to zero for $y \to y_{\rm c(KdS)}\doteq 0.059$, the maximum value of $y$ admitting black 