Central charge for AdS2 quantum gravity

Two-dimensional Maxwell-dilaton quantum gravity on AdS(2) with radius l and a constant electric field E is studied. In conformal gauge, this is equivalent to a CFT on a strip. In order to maintain consistent boundary conditions, the usual conformal diffeo-morphisms must be accompanied by a certain U(1) gauge transformation. The resulting conformal transformations are generated by a twisted stress tensor, which has a central charge c = 3kE(2)l(4)/4 where k is the level of the U(1) current. This is an AdS(2) analog of the Brown-Henneaux formula c = 3l/2G for the central charge of quantum gravity on AdS(3).