  Using Zwicky Transient Facility (ZTF) data, I noticed that MGAB-V240 =
PS1-3PI J185529.82+323017.8 showed two different states: regularly outbursting
state with a cycle length of 12 d and standstills. I found that the regularly
outbursting state was in fact a sequence of superoutburst and intervening
normal outbursts comprising a 12 d supercycle. During one of the
superoutbursts, superhumps with a period of 0.015824(9) d (=22.79 min) were
detected in the ZTF time-resolved data. This period and behavior have confirmed
that MGAB-V240 is an AM CVn-type object with the shortest known supercycle and
the second known AM CVn star showing genuine standstills. The standstills in
this system were interrupted by short drops and the system often brightened
after these drops. This phenomenon can be explained by the accumulation of the
transferred matter in the outer part of the disk during the drops. This
phenomenon favors a constant mass-transfer from the secondary combined with the
difficulty in maintaining the hot state in a helium disk rather than a
temporary decrease of the mass-transfer rate as the cause of these drops.
MGAB-V240 should be close to the border of the thermal instability of a helium
disk, and the observed superhump period agrees very well with the activity
sequence expected by the disk instability theory and the evolutionary sequence
of AM CVn stars.
