  We study the spectrum of open strings on AdS_2 branes in AdS_3 in an NS-NS
background, using the SL(2,R) WZW model. When the brane carries no fundamental
string charge, the open string spectrum is the holomorphic square root of the
spectrum of closed strings in AdS_3. It contains short and long strings, and is
invariant under spectral flow. When the brane carries fundamental string
charge, the open string spectrum again contains short and long strings in all
winding sectors. However, branes with fundamental string charge break half the
spectral flow symmetry. This has different implications for short and long
strings. As the fundamental string charge increases, the brane approaches the
boundary of AdS_3. In this limit, the induced electric field on the worldvolume
reaches its critical value, producing noncommutative open string theory on
AdS_2.
