A new class of spatially homogeneous 4D string backgrounds

A new class of spatially homogeneous 4D string backgrounds, the X(d\rightarrow) according to a recent classification, is presented and shown to contain only five generic types. In contrast to the case of X(d\uparrow) (which contains as a subclass all possible FRW backgrounds), exact SO(3) isotropy is always broken in the X(d\rightarrow) class. This is due to the H-field, whose dual is necessarily along a principal direction of anisotropy. Nevertheless, FRW symmetry can be attained asymptotically for Bianchi-types I and VII_0 in a rather appealing physical context. Other aspects of the solutions found for types X=I,II,III,VI_{-1}, and of the VII_0 case are briefly discussed.