Enhanced asymptotic symmetry algebra of AdS3

A generalization of the Brown-Henneaux boundary conditions is introduced for pure gravity with negative cosmological constant in 3 dimensions. This leads to new degrees of freedom and to an enhancement of the symmetry algebra. Up to the zero modes, it consists of two copies of the semi-direct product of a Virasoro algebra with a U(1) current algebra. The associated surface charge algebra now contains three non-zero central charges: the two usual Brown-Henneaux central charges and one new quantity.