F^4 Terms in N=4 String Vacua
  We discuss $F_{\mu\nu}^4$ terms in torroidal compactifications of type-I and heterotic SO(32) string theory. We give a simple argument why only short BPS multiplets contribute to these terms at one loop, and verify heterotic-type-I duality to this order. 
C. Bachas (Ecole Polytechnique) and E. Kiritsis (CERN)