Flat connections from flat gerbes
  We discuss some aspects of heterotic-Type I duality. We focus on toroidal compactification, with special attention for the topology of the gauge group, and the topology of the bundle. We review the arguments leading to a classification of Spin(32)/Z_2-bundles over tori, suitable for string compactifications. A central role is played by n-gerbes with connection, a generalization of bundles with connection. 
Arjan Keurentjes