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The ongoing Zwicky Transient Facility (ZTF) survey is generating a massivealert rate from a variety of optical transients and variable stars, which arebeing filtered down to subsets meeting user-specified criteria by brokersystems such as ANTARES. In a beta implementation of the algorithm of Soraisamet al. (2020) on ANTARES, we flagged AT 2020iko from the ZTF real-time alertstream as an anomalous source. This source is located close to a red extendedSDSS source. In the first few epochs of detection, it exhibited a V-shapedbrightness profile, preceded by non-detections both in ZTF and in ASASSNextending to 2014. Its full light curve shows a precursor event, followed by amain superoutburst and at least two rebrightenings. A low-resolution spectrumof this source points to a dwarf nova (DN) nature. Although some of thefeatures of AT 2020iko indicate an SU UMa-type DN, its large amplitude,presence of rebrightenings, and inferred supercycle period of > 6 yr are infavor of AT 2020iko being a new WZ Sge-type dwarf nova candidate, a subset ofrare DNe consisting of extreme mass-ratio (< 0.1) binaries with orbital periodaround the period minimum. AT 2020iko's precursor event brightened by 6.5 mag,while its decay spanned 3-5 mag. We speculate this superoutburst is associatedwith a less expanded accretion disk than in typical superoutbursts in WZ Sgesystems, with the large depth of the precursor decay implying an extremelysmall mass-ratio. To the best of our knowledge, such a precursor event has notbeen recorded for any DN. This result serves to demonstrate the efficacy of ourreal-time anomaly search algorithm.

AT 2020iko: a WZ Sge-type DN candidate with an anomalous precursor event