String Vertex Operators and Cosmic Strings
  We construct complete sets of (open and closed string) covariant coherent state and mass eigenstate vertex operators in bosonic string theory. This construction can be used to study the evolution of fundamental cosmic strings as predicted by string theory, and is expected to serve as a self-contained prototype toy model on which realistic cosmic superstring vertex operators can be based on. It is also expected to be useful for other applications where massive string vertex operators are of interest. We pay particular attention to all the normalization constants, so that these vertices lead directly to unitary S-matrix elements. 
Dimitri Skliros and Mark Hindmarsh
hep-th