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A new mechanism for the formation of binary systems through the fragmentation of a protostellar disc is presented. This process requires a strongly rotationally unstable (to both m = 2 and m = 1 modes), central accreting object surrounded by a rotationally supported disc of gas which is subject to continuing infall. The central instability becomes off-centre due to the growth of the m = 1 mode. The interaction of the spiral arms thus driven into the disc, with the continued infall, gives rise to the formation of a self-gravitating secondary body in orbit around the first.
I. A. Bonnell (1994) studied this question.