  In this paper we discuss some aspects of N=1 type I-heterotic string duality
in four dimensions. We consider a particular example of a (weak-weak) dual pair
where on the type I side there are only D9-branes corresponding to perturbative
heterotic description in a certain region of the moduli space. We match the
perturbative type I and heterotic tree-level massless spectra via giving
certain scalars appropriate vevs, and point out the crucial role of the
perturbative superpotential (on the heterotic side) for this matching. We also
discuss the role of anomalous U(1) gauge symmetry present in both type I and
heterotic models. In the perturbative regime we match the (tree-level) moduli
spaces of these models. Since both type I and heterotic models can be treated
perturbatively, we are able to discuss a dictionary that in generic models maps
type I description onto heterotic one, and vice-versa. Finally, we discuss
possible directions to study perturbative quantum corrections to the moduli
space, as well as outline ways to learn about the non-perturbative effects in
both descriptions.
