Massive fields dynamics in open bosonic string theory
  We consider the theory of open bosonic string in massive background fields. The general structure of renormalization is investigated. A general covariant action for a string in background fields of the first massive level is suggested and its symmetries are described. Equations of motion for the background fields are obtained by demanding that the renormalized operator of the energy-momentum tensor trace vanishes. 
I.L. Buchbinder, V.A. Krykhtin, V.D. Pershin