We report on a discovery of ” negative ” superhumps during the 2011 January superoutburst of ER UMa .
During the superoutburst which started on 2011 January 16 , we detected negative superhumps having a period of 0.062242 ( 9 ) d , shorter than the orbital period by 2.2 % .
No evidence of positive superhumps was detected during this observation .
This finding indicates that the disk exhibited retrograde precession during this superoutburst , contrary to all other known cases of superoutbursts .
The duration of this superoutburst was shorter than those of ordinary superoutbursts and the intervals of normal outbursts were longer than ordinary ones .
We suggest a possibility that such unusual outburst properties are likely a result of the disk tilt , which is supposed to be a cause of negative superhumps : the tilted disk could prevent the disk from being filled with materials in the outmost region which is supposed to be responsible for long-duration superoutbursts in ER UMa-type dwarf novae .
The discovery signifies the importance of the classical prograde precession in sustaining long-duration superoutbursts .
Furthermore , the presence of pronounced negative superhumps in this system with a high mass-transfer rate favors the hypothesis that hydrodynamical lift is the cause of the disk tilt .