The Dixmier-Douady Class of Groupoid Crossed Products
  We give a formula for the Dixmier-Douady class of a continuous-trace groupoid crossed product that arises from an action of a locally trivial, proper, principal groupoid on a bundle of elementary $C^*$-algebras that satisfies Fell's condition. 
Paul S. Muhly (University of Iowa), Dana P. Williams (Dartmouth   College)