On surface states and star-subalgebras in string field theory
  We elaborate on the relations between surface states and squeezed states. First, we investigate two different criteria for determining whether a matter sector squeezed state is also a surface state and show that the two criteria are equivalent. Then, we derive similar criteria for the ghost sector. Next, we refine the criterion for determining whether a surface state is in H_{\kappa^2}, the subalgebra of squeezed states obeying [S,K_1^2]=0. This enables us to find all the surface states of the H_{\kappa^2} subalgebra, and show that it consists only of wedge states and (hybrid) butterflies. Finally, we investigate generalizations of this criterion and find an infinite family of surface states subalgebras, whose surfaces are described using a "generalized Schwarz-Christoffel" mapping. 
Ehud Fuchs and Michael Kroyter