  IW And stars are a recently recognized subgroup of dwarf novae which are
characterized by (often repetitive) slowly rising standstills terminated by
brightening, but the exact mechanism for this variation is not yet identified.
We have identified BO Cet, which had been considered as a novalike cataclysmic
variable, as a new member of IW And stars based on the behavior in 2019-2020.
In addition to this, the object showed dwarf nova-type outbursts in 2020-2021,
and superhumps having a period 7.8% longer than the orbital one developed at
least during one long outburst. This object has been confirmed as an SU
UMa-type dwarf nova with an exceptionally long orbital period (0.1398 d). BO
Cet is thus the first cataclysmic variable showing both SU UMa-type and IW
And-type features. We obtained a mass ratio (q) of 0.31-0.34 from the
superhumps in the growing phase (stage A superhumps). At this q, the radius of
the 3:1 resonance, responsible for tidal instability and superhumps, and the
tidal truncation radius are very similar. We interpret that in some occasions
this object showed IW And-type variation when the disk size was not large
enough, but that the radius of the 3:1 resonance could be reached as the result
of thermal instability. We also discuss that there are SU UMa-type dwarf novae
above q=0.30, which is above the previously considered limit (q~0.25) derived
from numerical simulations and that this is possible since the radius of the
3:1 resonance is inside the tidal truncation radius. We constrained the mass of
the white dwarf larger than 1.0Msol, which may be responsible for the IW
And-type behavior and the observed strength of the He II emission. The exact
reason, however, why this object is unique in that it shows both SU UMa-type
and IW And-type features is still unsolved.
