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A<sc>bstract</sc> Orientifold planes play a crucial role in flux compactifications of string theory, and we demonstrate their deep connection to achieving scale-separated solutions. Specifically, we show that when an orientifold plane contributes at leading order to the non-zero value of the scalar potential, then either the weak coupling limit or the large volume limit implies scale separation, meaning that the Kaluza-Klein tower mass decouples from the inverse length scale of the lower-dimensional theory. Notably, in the supergravity limit such solutions are inherently scale-separated. This result is independent of the spacetime dimension and the dimensionality of the Op-plane as long asp <7. Similarly, we show, extending previous results, that parametric scale separation is not possible for isotropic compactifications with a leading curvature term that generically arise in the AdS/CFT context. We classify all possible flux compactification setups in both type IIA and type IIB string theory for Op-planes with 2 ≤p≤ 6 and present their universal features. While the parametrically controlled scale-separated solutions are all AdS, we also find setups that allow for dS vacua. We prove that flux quantization prevents these dS vacua in isotropic compactifications from arising in a regime of parametric control. 

A<sc>bstract</sc> We study supersymmetric scale-separated AdS₃flux vacua of massive IIA on G2 orbifolds with smeared orientifold planes. We consider two types of$$ {T}^7/{Z}_2^3 $$ $T^7/Z_2^3$orbifolds which, with appropriate flux choices, yield integer dual dimensions for the operators corresponding to the closed string scalar fields in the dual CFT. As with all other known examples, the dual conformal dimensions are only parametrically close to integer values. 

A<sc>bstract</sc> In this work, we investigate the properties of string effective theories with scalar field(s) and a scalar potential. We first claim that in most examples known, such theories aremultifield, with at least 2 non-compact field directions; the few counter-examples appear to be very specific and isolated. Such a systematic multifield situation has important implications for cosmology. Characterising properties of the scalar potentialVis also more delicate in a multifield setting. We provide several examples of string effective theories withV> 0, where the latter admits an asymptotically flat direction along an off-shell field trajectory: in other words, there exists a limit$$\widehat{\varphi }\to \infty $$for which$$\frac{\left|{\partial }_{\widehat{\varphi }}V\right|}{V}\to 0$$. It is thus meaningless to look for a lower bound to this single field quantity in a multifield setting; the complete gradient ∇Vis then better suited. Restricting to on-shell trajectories, this question remains open, especially when following the steepest descent or more generally a gradient flow evolution. Interestingly, single field statements in multifield theories seem less problematic forV< 0.