2 G M}{G M}\right)\varrho .$$ (Notice that considering possibility of $R < 2 G M$, we obtain a limit on the negative tension.)

The line element of the external geometry is given by the metric coefficients $A^{+}(r)$, $B^{+}(r)$ that are determined by $$\left( A^{+}(r)\right)^{2} = \left( B^{+}(r)\right)^{-2} = 1 - \frac{2G\mathcal{M}}{r} + \frac{q}{r^2},$$ where, due to the matching conditions on the neutron star surface, i.e., $A^{-}(R) = A^{+}(R)$, $B^{-}(R) = B^{+}(R)$, the external mass parameter $\mathcal{M}$ and external tidal charge parameter $q$ are related to the internal geometry parameters $\varrho$, $\lambda$ (and $M$) by the relations

$$q = -3G M R\frac{\varrho}{\lambda}$$

$$\mathcal{M} = M \left(1-\frac{\varrho}{\lambda}\right).$$

For $\lambda > 0$, the tidal charge $q < 0$ and $\mathcal{M} < M$, while for $\lambda < 0$, there is $q > 0$ and $\mathcal{M} > M$. Notice that for $\lambda > 0$, the condition $\varrho < \lambda$ has to be satisfied in order to have $\mathcal{M} > 0$.

For our purposes, it is convenient to express the internal spacetime coefficients using the parameters of the external spacetime that can be directly determined from observations of accretion and optical phenomena in vicinity of the neutron stars. Since the matching conditions imply the relations $$\label{EQq}
      