Minimum mass of the in-spiralling object, M_X,min which is needed to expel the envelope during a CE evolution with α = 1, for a given donor star radius as indicated by the coloured lines, as a function of ZAMS mass of the donor star, M_ZAMS for Z = Z_⊙/ 50 (upper panel) and Z = Z_MW (lower panel). R_max is the maximum radial extent during the stellar evolution. The grey band between 1.17 and 1.56M_⊙ indicates masses of NSs observed in double NS systems (Martinez et al. 2015).