Janus and J-fold solutions in type IIB supergravity

I discuss recent supergravity constructions of type IIB holographic interfaces in four-dimensional $\mathcal{N}=1$ and $\mathcal{N}=4$ field theories. I explain how each holographic interface can be compactified on a circle with an SL$(2,\mathbf{Z})$ monodromy leading to a novel AdS${}_4$ supergravity solution. These AdS${}_4$ backgrounds are argued to be dual to so-called J-fold superconformal field theories in three dimensions.