Genuine standstill in the AM CVn star CR Boo

Abstract

CR Boo is one of the brightest and most famous AM CVn stars showing dwarf nova-type outbursts. Previous studies showed different modes of outbursts in this object ranging from the one equivalent to a hydrogen-rich ER UMa star or WZ Sge star to a low-amplitude oscillating state. We for the first time identified a bona fide standstill in this object in 2022 and we consider that CR Boo is a helium analog of Z Cam stars in addition to its SU UMa/ER UMa-type classification. The standstill lasted for ~60 d with variations typically less than 0.2 mag and ended with fading. This standstill was not preceded by a superoutburst and was different from a post-superoutburst phenomenon. The brightness after the standstill was similar to those after superoutbursts and the standstill appears to have acted like a superoutburst in effectively accreting the disk mass. The existence of a standstill in an AM CVn star be a challenge to theories of a helium disk or a degenerate secondary to explain how such a state could be maintained.