Discovery of a Peculiar SU UMa-Type Dwarf Nova ER Ursae Majoris

An ultraviolet excess object, ER Ursae Majoris (=PG 0943+521), was confirmed to be an SU UMa-type dwarf nova with a superhump period of 0.06549--0.06573 day. This dwarf nova shows a most unusual outburst light curve: superoutbursts occurring every only 43 d and lasting about 20 d, normal outbursts occurring every four days, and a full outburst amplitude of only 3.0 mag. The authors suggest that such an exceptional behavior may be the consequence of an extremely large mass-transfer rate of this system. The implications of this discovery on the origin of mass-transfer in cataclysmic variables and the disk-instability theory are briefly discussed.