Singlets in Supersymmetry
  It is argued that singlet extensions of the MSSM at the weak scale are indicative of either gauged-$R$ symmetry or target space duality in a string effective action at the Planck scale. The criteria used are satisfactory primordial nucleosynthesis, absence of fine-tuning, and absence of cosmological problems such as domain walls. Models which have a global discrete symmetry such as the NMSSM may only be accommodated within rather complicated cosmological scenarios which are also discussed. 
S.A.Abel