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Abstract A working model is proposed in which the outbursts of U Geminorum stars (or dwarf novae) may be caused by sudden gravitational energy release due to intermittent accretion of material onto the white dwarf component of the close binary from the surrounding disk. The intermittent accretion has been hypothesized to be triggered by some kind of instability within the disk. It is demonstrated that this accretion model naturally explains amplitudes of outbursts and the period-amplitude relation for the outbursts. The standstills observed in Z Cam stars are then interpreted as a more or less continuous accretion of matter from the disk.
Yoji Osaki (Mon,) studied this question.