Heterotic versus type I

I compare the (prototype) calculations of special F4 and R4 terms in the effective actions of the Spin(32)Z2 heterotic and type I string theories, compactified on S1 and T2. Besides checking duality, this elucidates the quantitative rules of D-brane calculus. I explain in particular (a) why D-branes do not run in loops, and (b) how their instanton contributions arise from orbifold fixed points of their moduli space.