  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.
