  We investigate in detail the critical $N{=}2$ fermionic string with and
without a global ${\bf Z}_2$ twist. An analysis of BRST cohomology shows that
twisted sectors contain massless `spacetime' fermions which are {\it
non-local\/} with respect to the standard massless boson. However, two distinct
GSO projections exist, one (untwisted) retaining merely the usual boson and its
spectral-flow partner, the other (twisted) yielding two fermions and one boson,
on the massless level. The corresponding chiral BRST-invariant vertex operators
are constructed in certain pictures, and their fusion and picture-changing are
investigated, including the construction of inverse picture-changing operators.
The $N{=}2$ `spacetime supersymmetry' generators are {\it null\/} operators,
since the twisted massless states fail to interact. The untwisted three- and
four-point functions are recalculated at tree-level.

High Energy Physics - Theory