M & mʹs Original Research Article

We consider M-theory 5-branes with compact transverse dimensions. In certain limits the theory on the 5-brane decouples and defines “little string theories” in 5 + 1 dimensions. We show that the familiar structure of IIA/IIB, M-, F-theory in 10, 11, 12 dimensions, respectively, has a perfect parallel in a theory of strings and membranes in 6, 7, 8 dimensions. We call these theories a/b-, m-, f-theories. They have a coupling constant but no gravity. This construction clarifies some mysteries in F-theory and leads to several speculations about the phase structure of M-theory.