  The ongoing Zwicky Transient Facility (ZTF) survey is generating a massive
alert rate from a variety of optical transients and variable stars, which are
being filtered down to subsets meeting user-specified criteria by broker
systems such as ANTARES. In a beta implementation of the algorithm of Soraisam
et al. (2020) on ANTARES, we flagged AT 2020iko from the ZTF real-time alert
stream as an anomalous source. This source is located close to a red extended
SDSS source. In the first few epochs of detection, it exhibited a V-shaped
brightness profile, preceded by non-detections both in ZTF and in ASASSN
extending to 2014. Its full light curve shows a precursor event, followed by a
main superoutburst and at least two rebrightenings. A low-resolution spectrum
of this source points to a dwarf nova (DN) nature. Although some of the
features of AT 2020iko indicate an SU UMa-type DN, its large amplitude,
presence of rebrightenings, and inferred supercycle period of > 6 yr are in
favor of AT 2020iko being a new WZ Sge-type dwarf nova candidate, a subset of
rare DNe consisting of extreme mass-ratio (< 0.1) binaries with orbital period
around 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 associated
with a less expanded accretion disk than in typical superoutbursts in WZ Sge
systems, with the large depth of the precursor decay implying an extremely
small mass-ratio. To the best of our knowledge, such a precursor event has not
been recorded for any DN. This result serves to demonstrate the efficacy of our
real-time anomaly search algorithm.
