In  they write that
"Boucher et al.  found quite generally that $(0, 2)$ world-sheet SUSY ensures the existence of $N = 1$ spacetime SUSY."
But this reference "" they cite just says: "W. Boucher, D. Friedan, A. Kent, S.H. Shenker and E. Witten, in preparation". I did not find a paper with these authors, but it is hard without knowing the title or the magazine/journal.
Does anyone know what paper they are referring to?
 Banks, Thomas, et al. "Phenomenology and conformal field theory or can string theory predict the weak mixing angle?." Nuclear Physics B 299.3 (1988): 613-626.