F-theory with zeroth-quantized ghosts
  F-theory in its most general sense should be a theory defined on a worldvolume of higher dimension than the worldsheet, that reproduces string results perturbatively but includes nonperturbative supergravity solutions at the first-quantized level. This implies that in some sense it should contain the same oscillator modes as the string but an enlarged set of zero-modes. In this paper we concentrate on the higher-dimensional properties of the worldvolume (rather than those of spacetime): "Ghost" dimensions are added to the worldvolume, as might be expected in a "zeroth-quantized" approach to the constraints on its higher bosonic dimensions, by adding equal numbers of bosonic and fermionic dimensions to the worldsheet. 
W. Siegel
hep-th