Einstein-Maxwell theory is not only covariant under diffeomorphisms but also is under U(1) gauge trans-formations. We introduce a combined transformation constructed out of diffeomorphism and U(1)gauge transformation. We show that symplectic potential, which is defined in covariant phase space method, is not invariant under combined transformations. In order to deal with that problem, following Donnelly and Freidel proposal [19], we introduce new fields. In this way, phase space and consequently symplectic potential will be extended. We show that new fields produce edge modes. We consider surface-preserving symmetries and we show that the group of surface-preserving symmetries is semi-direct sum of 2-dimensional diffeo-morphism group on a spacelike codimension two surface with SL(2, R) and U(1). Eventually, we deduce that the Casimir of SL(2, R)is the area element, similar to the pure gravity case [19].
