Harmless R-parity violation from Z_{12-I} compactification of E_8 X E_8'   heterotic string
  In a recent $\Z_{12-I}$ orbifold model, an approximate $Z_2$ symmetry which forbids the baryon number violating operators up to sufficiently high orders is found. The dimension-4 $\Delta B\ne 0$ operators of the MSSM fields occur at dimension 10. The effective dimension-5 $\Delta B\ne 0$ operators derived from these are harmless if some VEVs of neutral singlets are O(1/10) times the string scale. The main reason for forbidding these $\Delta B\ne 0$ operators up to such a high order is the large order N=12 of $\Z_N$ since the $H$-momentum rule is $(-1,1,1)$ mod (12, 3, 12). For a lower order $N<12$, the $\Delta B\ne 0$ operators would appear at lower dimensions. 
Ian-Woo Kim, Jihn E. Kim, Bumseok Kyae