  We show that closed string states in bosonic string field theory are encoded
in the cyclic cohomology of cubic open string field theory (OSFT) which, in
turn, classifies the deformations of OSFT. This cohomology is then shown to be
independent of the open string background. Exact elements correspond to closed
string gauge transformations, generic boundary deformations of Witten's
3-vertex and infinitesimal shifts of the open string background. Finally it is
argued that the closed string cohomology and the cyclic cohomology of OSFT are
isomorphic to each other.
