6b = 0.$$ This leads to a cubic equation relative to $R$ which gives one solution that is relevant for $b\leq 1$ and reads $$\begin{aligned}
       R & = & \frac{1}{4} \left(\frac{(b-9) (b-1)}{\sqrt[3]{8 \sqrt{(b-1)^2 b^3}+b \left[b (b+17)-45\right]+27}} + \right. \nonumber \\
        &   & + \left.\sqrt[3]{8 \sqrt{(b-1)^2 b^3}+b \left[b (b+17)-45\right]+27}+b+3\right).
    \end{aligned}$$ The solution is depicted on Figure \[FIGzone\] as the part of $R_\mathrm{min}(b)$ denoted by $p \rightarrow \infty$. For $b = 0$, we arrive at the standard condition $R > \frac{9}{4}$ (see [@Stu-Tor-Hle-Urb:2009:]). For $b > 1$ the relevant limit is given by the condition of positiveness of the central pressure. It reads $$R_\mathrm{min}(b| b > 1) = \frac{2}{3}b$$ and is denoted by $p \rightarrow 0$. For smaller $R$ the central pressure is not positive.

Note that for $3/4 < b < 1$ there is a region of surface radii given by the condition $2b/3 < R < 1-\sqrt{1-b}$ where $p(r = 0)$ is positive. However, the star surface is located under the inner horizon of the external spacetime that belongs to the black hole type R-N spacetime. Such a configuration has to be hidden under the inner horizon of the external spacetime and is irrelevant for our considerations.

![Classification of the 