Open branes in space-time non-commutative little string theory

We conjecture the existence of two new non-gravitational six-dimensional string theories, defined as the decoupling limit of NS5-branes in the background of near-critical electrical two- and three-form RR fields. These theories are space-time non-commutative Little String Theories with open branes. The theory with (2, 0) supersymmetry has an open membrane in the spectrum and reduces to OM theory at low energies. The theory with (1, 1) supersymmetry has an open string in the spectrum and reduces to (5 + 1)-dimensional NCOS theory for weak NCOS coupling and low energies. The theories are shown to be T-dual with the open membrane being T-dual to the open string. The theories therefore provide a connection between (5 + 1)-dimensional NCOS theory and OM theory. We study the supergravity duals of these theories and we consider a chain of dualities that shows how the T-duality between the two theories is connected with the S-duality between (4 + 1)-dimensional NCOS theory and OM theory. (C) 2001 Elsevier Science B.V. All rights reserved.