  We determine a consistent phase space for a theory consisting in the
Einstein-Hilbert action coupled to matter fields (dilaton, one-form, two-form)
and containing three-dimensional black strings (the Horne-Horowitz solution and
generalizations thereof). The theory at hand is the low energy effective action
for the bosonic sector of heterotic string theory. We find a consistent set of
boundary conditions whose algebra of asymptotic charges consist in a single
Virasoro algebra supplemented by three global $u(1)$ generators. We also
discuss the thermodynamics of the zero-mode solutions and point out some
peculiar features of this system.
