  We formulate a twisted version of the conjectured duality between heterotic
and type I string theories. Our formulation relates the chiral part of the
heterotic string with a type I topological B-model on a Calabi-Yau five-fold.
We provide a non-trivial check of this duality by showing that certain
infinite-dimensional Lie algebras of global gauge transformations built from
each theory are isomorphic. Matching the structure constants of the Lie
algebras involves a detailed analysis of one-loop corrections on the type I
side.
