result $a_2$ is bounded by the O3 plane charge. In the concrete examples considered in [@BVW] the largest value of the RR charge arising from an O3 plane was 12. As a result Minkowski space solutions only exist if the imaginary part of the complex structure is small like for example in the solutions found in [@BVW]. In the type IIA mirror these are solutions at small volume of the internal geometry, for which no perturbative control is expected. In the following we will see that AdS solutions do exist in the large complex structure limit.

Constraints for AdS type solutions
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Supersymmetric flux configurations which allow a negative cosmological constant in the external space-time are the solutions of $$D_\tau W=D_t W=0.$$ Since the vanishing of $W$ is no longer required, solutions of AdS type are less constrained. It is useful to first rewrite the superpotential in a more practical manner. For this take into account that the equation $D_{\tau}W=0$ allows us to write $\tau$ in the form[^6] $$\eqlabel{tau}
\tau = \frac 12 {W_{RR}\over W_{NS}}(3-e^{i \varphi}).$$ Inserting this form of $\tau$ into the superpotential we get a representation of the superpotential in terms of $W_{RR}$ up to a phase $$\label{superpot} 