Dunbar, D., Bern, Z., & Dunbar, D. (1990). Four-dimensional type I superstrings withN < 4 supergravity. Physics Letters B, 242(2), pp. 175-184. doi:10.1016/0370-2693(90)91455-K

Dunbar, David, Z. Bern, and D.C Dunbar. "Four-dimensional Type I Superstrings WithN < 4 Supergravity." Physics Letters B 242, no. 2 (1990): 175-184.

Dunbar, David, Z. Bern, and D.C Dunbar. "Four-dimensional Type I Superstrings WithN < 4 Supergravity." Physics Letters B 242.2 (1990): 175-184.