  SDSS J183131.63+420220.2 is an AM CVn-type cataclysmic variable. Using
Asteroid Terrestrial-impact Last Alert System (ATLAS) and Zwicky Transient
Facility (ZTF) data, I found that this object is actually a helium dwarf nova,
which experienced a long (~6 yr) standstill (2017 to 2022). The object is
currently (in 2023) in ER UMa-type state with supercycles of 20-30 d and large
duty cycles exceeding 0.5. This object is the second known star among AM CVn
stars having characteristics of ER UMa and Z Cam types. The long duration of
the standstill phase is outstanding among helium dwarf novae. These
observations indicate that the accretion disk in SDSS J183131.63+420220.2 is
very close to thermal stability. I detected a period of 0.01602343(1) d in the
ZTF data, which can be the orbital one. Combined with the case of MGAB-V240,
the limit of thermal stability of the disks in AM CVn stars appears to be
located around the orbital period of 0.0158-0.0160 d.
