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IW And stars are a recently recognized subgroup of dwarf novae which arecharacterized by (often repetitive) slowly rising standstills terminated bybrightening, but the exact mechanism for this variation is not yet identified.We have identified BO Cet, which had been considered as a novalike cataclysmicvariable, 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 atleast during one long outburst. This object has been confirmed as an SUUMa-type dwarf nova with an exceptionally long orbital period (0.1398 d). BOCet is thus the first cataclysmic variable showing both SU UMa-type and IWAnd-type features. We obtained a mass ratio (q) of 0.31-0.34 from thesuperhumps in the growing phase (stage A superhumps). At this q, the radius ofthe 3:1 resonance, responsible for tidal instability and superhumps, and thetidal truncation radius are very similar. We interpret that in some occasionsthis object showed IW And-type variation when the disk size was not largeenough, but that the radius of the 3:1 resonance could be reached as the resultof thermal instability. We also discuss that there are SU UMa-type dwarf novaeabove q=0.30, which is above the previously considered limit (q~0.25) derivedfrom numerical simulations and that this is possible since the radius of the3:1 resonance is inside the tidal truncation radius. We constrained the mass ofthe white dwarf larger than 1.0Msol, which may be responsible for the IWAnd-type behavior and the observed strength of the He II emission. The exactreason, however, why this object is unique in that it shows both SU UMa-typeand IW And-type features is still unsolved.

BO Ceti: Dwarf Nova Showing Both IW And and SU UMa-Type Features