  We consider the simplest possible setting of non abelian twist fields which
corresponds to $SU(2)$ monodromies. We first review the theory of
hypergeometric function and of the solutions of the most general Fuchsian
second order equation with three singularities. Then we solve the problem of
writing the general solution with prescribed $U(2)$ monodromies. We use this
result to compute the classical string solution corresponding to three $D2$
branes in $R^4$. Despite the fact the configuration is supersymmetric the
classical string solution is not holomorphic. Using the equation of motion and
not the KLT approach we give a very simple expression for the classical action
of the string. We find that the classical action is not proportional to the
area of the triangle determined by the branes intersection points since the
solution is not holomorphic. Phenomenologically this means that the Yukawa
couplings for these supersymmetric configurations on non factorized tori are
suppressed with respect to the factorized case.
