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We report photometric and spectroscopic observations of the eclipsing dwarf nova IY Ursae Majoris (=Takamizawa V85). During its 2000 January superoutburst, the star flashed superhumps with a period of 0.07583 day, 2.6% longer than the true orbital period. These waves rumbled through the light curve for ∼20 days. They appeared compatible with their common interpretation as the lower precessional sideband of the orbital clock: at a frequency ω_o-Ω, where ω_o is the orbital frequency and Ω is the accretion disk's assumed precessional frequency. But the power spectrum of the superhumps showed additional complexity, including signals at 3ω_o-Ω, 4ω_o-Ω, and 5ω_o-Ω. The latter probably arise from gravitational perturbation of more complex but still resonant (e.g., spiral) structures in the disk.

During eruption, the light curve across eclipse showed a large, bright accretion disk. Timings of minima and eclipse contacts reveal that the disk was quite eccentric, with e=0.29±0.06, and that this eccentric shape moved around with the putative precession period of 2.9 days. As the eruption faded, the eclipses began to reveal the signature of the white dwarf and the hot spot at the disk's edge. ``Late'' superhumps raged at high amplitude for another ∼10 days, although the disk had seemingly contracted by ∼30%. Spectroscopy in quiescence showed strong doubled emission lines, typical of low-M dwarf novae, and an S-wave with a semiamplitude of 750 km.s^–1.

The traveling knife-edge of the secondary's limb made it possible to separate all the important light sources in quiescence: white dwarf, secondary star, hot spot, and accretion disk. The ingress and egress phases of the white dwarf eclipse were very brief, declining from ∼40 s to 25 s. The brightness of the white dwarf also varied, rapidly at first and then slowly. Eclipse measurements in quiescence lead to estimates for the fundamental parameters of the binary: q=0.13±0.02, M₂=0.12±0.02 M_☉, M₁=0.86±0.11 M_☉, and i=86°.8±1°.5. We estimate a distance of 190±60 pc. With a small distance, a high inclination, vigorous superhumps, bright emission lines, and clear lines of sight to the central object, IY UMa furnishes a promising new laboratory for studying accretion-disk physics.