Worldsheet instantons and a null string limit of Born–Infeld theory G.A. Goldin, N.E. Mavromatos and R.J. Szabo Eur. Phys. J. C, 53 4 (2008) 667-672 Published online: 08 December 2007 DOI: 10.1140/epjc/s10052-007-0474-5