  We study conformal field theory correlation functions relevant for string
diagrams with open strings that stretch between several parallel branes of
different dimensions. In the framework of conformal field theory, they involve
boundary condition changing twist fields which intertwine between Neumann and
Dirichlet conditions. A Knizhnik-Zamolodchikov-like differential equation for
correlators of such boundary twist fields and ordinary string vertex operators
is derived, and explicit integral formulas for its solutions are provided.
