Dunbar, D., Bern, Z., & Dunbar, D. (1988). Coupling of open to closed bosonic strings in four dimensions. Physics Letters B, 203(1-2), pp. 109-117. doi:10.1016/0370-2693(88)91580-8

Dunbar, David, Z. Bern, and D.C Dunbar. "Coupling of Open to Closed Bosonic Strings in Four Dimensions." Physics Letters B 203, no. 1-2 (1988): 109-117.

Dunbar, David, Z. Bern, and D.C Dunbar. "Coupling of Open to Closed Bosonic Strings in Four Dimensions." Physics Letters B 203.1-2 (1988): 109-117.