Superoutburst of CR Bootis: Estimation of mass ratio of a typical AM CVn star by stage A superhumps

We report on two superoutbursts of the AM CVn-type object CR Boo in 2014 April-March and 2015 May-June. A precursor outburst accompanied both of these superoutbursts. During the rising branch of the main superoutburst in 2014, we detected growing super-humps (stage A superhumps) whose period was 0.017669(24) d. Assuming that this period reflects the dynamical precession rate at the radius of the 3: 1 resonance, we could estimate the mass ratio (q = M-2/M-1) of 0.101(4) by using the stage A superhump period and the orbital period of 0.0170290(6) d. This mass ratio is consistent with that expected from the theoretical evolutionary model of AM CVn-type objects. The detection of precursor outbursts and stage A superhumps is the second case in AM CVn-type objects. There are two interpretations of the outbursts of AM CVn-type objects. One is a dwarf nova (DN) outbursts analogy, which suggets that the outbursts are caused by thermal and tidal instabilities. Another is the VY Scl-type variation, which suggests that the outbursts are caused by the variation of the mass-transfer rate of the secondary. This detection of the superhump variations strongly supports the former interpretation.