BO Ceti: Dwarf nova showing both IW And-type and SU UMa-Type features

Abstract

Abstract 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 its behavior in 2019–2020. In addition to this, the object showed dwarf nova-type outbursts in 2020–2021, and superhumps that had periods $7.8\%$ longer than the orbital one developed during at least 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 a 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 (∼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 1.0 M⊙, 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.