Fundamental strings in Dp-Dq brane systems

We study conformal field theory correlation functions relevant For string d iagrams with open strings that stretch between several parallel branes of d ifferent dimensions. In the framework of conformal field theory, they invol ve boundary condition changing twist fields which intertwine between Neuman n and Dirichlet conditions, A Knizhnik-Zamolodchikov-like differential equa tion for correlators of such boundary twist fields and ordinary string vert ex operators is derived, and explicit integral formulas for its solutions a re provided, (C) 2000 Elsevier Science B.V. All rights reserved.