Interaction of branes in presence of internal gauge fields is considered by
using the boundary state formalism. This approach enables us to consider t
he problems that are not easily accessible to the canonical approach via op
en strings. The effects of compactification of some of the dimensions on to
ri are also discussed. Also we study the massless state contribution on thi
s interaction. (C) 1999 Published by Elsevier Science B.V. All rights reser
ved.