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The first term of the action is the free boson conformal field theory. For the curvature coupling, insert the supplied first variation of $\sqrt{g}{R}^{(2)}$, integrate by parts twice and discard the boundary term. Its metric variation is the stress-tensor improvementin the conventions of the question. In a locally flat complex coordinate, the holomorphic component of the complete stress-energy tensor is thereforeas required. Thus the coupling $q\int \sqrt{g}X{R}^{(2)}$ makes $X$ a background-charge scalar field, equivalently a worldsheet coordinate in a linear dilaton conformal field theory.