Superoutbursts and Positive Superhumps Occurred during the Standstill of a Z Cam-type Dwarf Nova

Abstract

Dwarf novae (DNe) are semidetached binaries, where a white dwarf accretes material from a cool main-sequence companion via an accretion disk, and are known for their intermittent outbursts, making them key systems for studying accretion physics. The accumulation of large survey data sets has challenged traditional models, which assumed that the disk remains hot and cannot produce superoutbursts during the standstill of a Z Camelopardalis (Z Cam)-type DN and that superoutbursts require a mass ratio of q = M2/M1 ≤ 0.25–0.33. Here we report the detection of superoutbursts and positive superhumps (PSHs) during a standstill in the Z Cam-type star AT Cnc with a mass ratio larger than 0.33. Notably, the PSHs evolve gradually before the superoutburst begins, suggesting that an eccentric, precessing disk forms first, with the superoutburst occurring as the disk radius continues to expand. These findings provide the first detailed observational evidence of superoutbursts and PSHs occurring during standstill, offering important new insights into the classification of DNe and the underlying mechanisms of outbursts.