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Sliding Surface Charges on AdS$_3$

We consider Einstein gravity on a patch of AdS$_3$ spacetime between two radii $r_1, r_2$. We compute surface charges and their algebra at an arbitrary radius $r$ such that it reduces to a given set of surface charges at $r_1, r_2$. The $r$-dependent charges become integrable upon addition of an appropriate boundary $Y$-term. We observe that soft excitations at each boundary are independent of those at the other boundary. We explicitly construct solution geometries which interpolate between these radii with specified surface charges. The interpolation is smooth provided that the mass and angular momentum measured at the two boundaries are equal.