  We give a general method to compute the expansion of topological tau
functions for Drinfeld-Sokolov hierarchies associated to an arbitrary untwisted
affine Kac-Moody algebra. Our method consists of two main steps: first these
tau functions are expressed as (formal) Fredholm determinants of the type
appearing in the Borodin-Okounkov formula, then the kernels for these
determinants are found using a reduced form of the string equation. A number of
explicit examples are given.
