Interpolating between open and closed strings - a BSFT approach
  We address the conjecture that at the tachyonic vacuum open strings get transformed into closed strings. We show that it is possible in the context of boundary string field theory to interpolate between the conventional open string theory, characterized by having the D25 brane as the boundary state, and an off-shell (open) string theory where the boundary state is identified with the closed string vacuum, where holomorphic and antiholomorphic modes decouple and where bulk vertex operator correlation functions are identical to those of the closed string. 
J. Ambjorn and R.A. Janik