Worldsheet instantons and coupling selection rules in heterotic   orbifolds
  We review recent results on string coupling selection rules for heterotic orbifolds, derived using conformal field theory. Such rules are the first step towards understanding the viability of the recently obtained compactifications with potentially realistic particle spectra. They arise from the properties of the worldsheet instantons that mediate the couplings, and include stringy effects that would seem 'miraculous' to an effective field theory observer. 
Susha L. Parameswaran and Ivonne Zavala
hep-th