Large-amplitude superhumps in ER ursae majoris during the earliest stage of a superoutburst

Time-resolved CCD photometry of a peculiar SU UMa-type dwarf nova, ER UMa, during the earliest stage of superoutbursts revealed the existence of large-amplitude (similar to 0.35 mag) superhumps. These superhumps appear at, or even during, the rise to the superoutburst maximum, and show a rapid decay of amplitudes with a time scale of a few days. After a period showing a low-amplitude variation, superhumps with more usual singly-peaked profiles appear in the later stage of the superoutburst. Such an early appearance of superhumps seems to contradict the current growth model of superhumps or tidal instability, whose growth rate has been shown to be proportional to the square of the binary mass ratio. Several possibilities to explain this anomaly are also briefly discussed, as well as a comparison with other early stage superhumps in WZ Sge stars.