  It is shown how two-dimensional states corresponding to D-branes arise in
orbifolds of topologically massive gauge and gravity theories. Brane vertex
operators naturally appear in induced worldsheet actions when the
three-dimensional gauge theory is minimally coupled to external charged matter
and the orbifold relations are carefully taken into account. Boundary states
corresponding to D-branes are given by vacuum wavefunctionals of the gauge
theory in the presence of the matter, and their various constraints, such as
the Cardy condition, are shown to arise through compatibility relations between
the orbifold charges and bulk gauge invariance. We show that with a further
conformally-invariant coupling to a dynamical massless scalar field theory in
three dimensions, the brane tension is naturally set by the bulk mass scales
and arises through dynamical mechanisms. As an auxilliary result, we show how
to describe string descendent states in the three-dimensional theory through
the construction of gauge-invariant excited wavefunctionals.
