More on non-supersymmetric asymmetric orbifolds with vanishing cosmological constant

We explore various non-supersymmetric type ii string vacua constructed based on asymmetric orbifolds of tori with vanishing cosmological constant at the one loop. the string vacua we present are modifications of the models studied in [14], of which orbifold group is just generated by a single element. we especially focus on two types of modifications: (i) the orbifold twists include different types of chiral reflections not necessarily removing massless rarita-schwinger fields in the 4-dimensional space-time, (ii) the orbifold twists do not include the shift operator. we further discuss the unitarity and stability of constructed non-supersymmetric string vacua, with emphasizing the common features of them.