BRANES AT ORBIFOLDS VERSUS HANANY-WITTEN IN 6 DIMENSIONS

We reconstruct non-trivial 6d theories obtained by Blum and Intriligat or by considering IIB or SO(32) 5 branes at ALE spaces in the language of Hanany-Witten setups. Using ST duality we make the equivalence of the two approaches manifest, thereby uncovering several new T-duality relations between the group theoretic data describing the embedding of the instantonic 5 brane in the ALE and brane positions in the Hanany- Witten language. We construct several new 6d theories, which can be un derstood as arising on 5 branes in IIB orientifolds with oppositely ch arged orientifold planes recently introduced by Witten.