Superhumps in cataclysmic binaries. XII. CR bootis, a helium dwarf nova

CR Bootis is an enigmatic blue variable star with rapid photometric variations and a spectrum dominated by helium. It consists of two white dwarfs in close orbit, with a probable underlying binary period of 1471 s. For years we have marveled at the star's large nightly variations-ramping up or down at a rate of ∼ 0.1 mag/hr. An intensive photometry campaign in 1996 showed that this variability is cyclic with a quasiperiod of about 19 hr, and demonstrated the association of 1490-s photometric variations ("superhumps") with extended bright states ("superoutbursts"). During the superoutburst, the 1490-s signal initially decreased with Ṗ = -2×10-5, but then stabilized at 1487.29±0.02 s after ∼ 300-600 binary orbits. Spectroscopy reveals variably asymmetric absorption lines, with the asymmetry migrating on a probable period of 36 hr; this may be the period of accretion disk precession. Neither the helium composition, nor the degeneracy of the mass-losing component, nor the shortness of the period (all of the periods) seem to present any barrier to the star in being fully certifiable as a bona fide dwarf nova. Stabilization of the superhump period at such a low value (1487.29 s) favors a model in which period changes arise from eccentricity changes rather than mean radius changes in the disk. This naturally explains why decreasing period and decreasing amplitude are strongly linked in the superhumps of dwarf novae.