On string models with Scherk-Schwarz supersymmetry breaking

We construct a general class of chiral four-dimensional string models withScherk--Schwarz supersymmetry breaking, involving freely acting orbifolds. Thebasic ingredient is to combine an ordinary supersymmetry-preserving Z_Nprojection with a supersymmetry-breaking projection Z_M' acting freely on asubspace of the internal manifold. A crucial condition is that any generator ofthe full orbifold group Z_N x Z_M' must either preserve some supersymmetry oract freely in order to become irrelevant in some large volume limit. Tachyonsare found to be absent or limited to a given region of the tree-level modulispace. We find several new models with orthogonal supersymmetries preserved atdistinct fixed-points. Particular attention is devoted to an interesting Z_3 xZ_3' heterotic example.Comment: v2: Minor corrections and a reference added; v3: Title and abstract slightly changed, figure 2 corrected and a reference adde