Orientifold and F-theory Duals of CHL Strings

We present orientifold and F-theory duals of the heterotic string compactification constructed by Chaudhuri, Hockney and Lykken (CHL) which has the maximal supersymmetry but gauge group of reduced rank. The 8-dimensional dual is given by the Type IIA orientifold on the M\"{o}bius band. We show the non-trivial monodromy on the base induces non-simply laced gauge groups on the F-theory orbifolds dual to the CHL strings. The F-theory models dual to the CHL strings in six dimensions are examples of N=2 F-theory vacua. We discuss the other N=2 F-theory vacua in six dimensions.