  $AdS_3$ string theory in the stringy regime $k=(R_{AdS}/\ell_{str})^2 < 1$
provides a laboratory for the study of holography in which both sides of
AdS/CFT duality are under fairly good control. Worldsheet string theory is
solvable, and for closed strings the dual spacetime CFT is a deformation of a
symmetric product orbifold. Here we extend this construction to include open
strings by adding a probe D-string, described semi-classically by an $AdS_2$
D-brane in $AdS_3$. The dual defect or boundary conformal field theory (BCFT)
is again a deformed symmetric product, which now describes the Fock space of
long open and closed strings near the AdS boundary, with a boundary deformation
implementing the open/closed transition in addition to the symmetric product
${\mathbb Z}_2$ twist deformation that implements closed string
joining/splitting. The construction thus provides an explicit example of an
$AdS_3/BCFT_2$ duality.
