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Observation of ER UMa Stars

ER UMa stars are a recently recognized small subgroup of SU UMa-type dwarf novae, which are characterized by the extremely high outburst frequency and short (19--48 d) supercycles. From the current thermal-tidal disk instability scheme, they are considered to be high mass-transfer SU UMa-type dwarf novae, and comprise a link to "permanent superhumpers" below the period gap. They do not only provide an opportunity to test the applicability of thermal-tidal instability model but also pose problems on the origin of high mass-transfer in short orbital-period cataclysmic variables. A historical review of this subgroup and recent topics of ER UMa stars, the unique pattern of superhump evolution and the "helium ER UMa analog" (CR Boo), are also discussed.