SUPERHUMPS IN CATACLYSMIC BINARIES .7. CR BOOTIS, A HELIUM DWARF NOVA

CR Bootis is an enigmatic blue variable star with rapid photometric va riations and a spectrum dominated by helium. It consists of two white dwarfs in close orbit, with a probable underlying binary period of 147 1 s. For years we have marveled at the star's large nightly variations -ramping up or down at a rate of similar to 0.1 mag/hr. An intensive p hotometry campaign in 1996 showed that this variability is cyclic with a quasiperiod of about 19 hr, and demonstrated the association of 149 0-s photometric variations (''superhumps'') with extended bright state s (''superoutbursts''). During the superoutburst, the 1490-s signal in itially decreased with P = -2x10(-5), but then stabilized at 1487.29+/ -0.02 s after similar to 300-600 binary orbits. Spectroscopy reveals v ariably asymmetric absorption lines, with the asymmetry migrating on a probable period of 36 hr; this may be the period of accretion disk pr ecession, Neither the helium composition, nor the degeneracy of the ma ss-losing component, nor the shortness of the period (all of the perio ds) seem to present any barrier to the star in being fully certifiable as a bona fide dwarf nova, Stabilization of the superhump period at s uch a low value (1487.29 s) favors a model in which period changes ari se:from eccentricity changers rather than mean radius changes in the d isk. This naturally explains why decreasing period and decreasing ampl itude are strongly linked in the superhumps of dwarf novae.