Superhumps in cataclysmic binaries. XVIII. IY Ursae Majoris

We report photometric and spectroscopic observations of the eclipsing dwarf nova IY Ursae Majoris (=Takamizawa V85). During its 2000 January superoutb urst, 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 similar to 20 days. They appeared compatible with their common interpre tation as the lower precessional sideband of the orbital clock: at a freque ncy omega (0) - Omega, where omega (0) is the orbital frequency and Omega i s the accretion disk's assumed precessional frequency. But the power spectr um of the superhumps showed additional complexity, including signals at 3 o mega (0) - Omega, 4 omega (0) - Omega, and 5 omega (0) - Omega. The latter probably arise from gravitational perturbation of more complex but still re sonant (e.g. spiral) structures in the disk. During eruption, the light curve across eclipse showed a large, bright accr etion 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 mov ed 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 th e hot spot at the disk's edge. "Late" superhumps raged at high amplitude fo r another similar to 10 days, although the disk had seemingly contracted by similar to 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 separa te all the important light sources in quiescence: white dwarf, secondary st ar, hot spot, and accretion disk. The ingress and egress phases of the whit e dwarf eclipse were very brief, declining from similar to 40 s to 25 s. Th e brightness of the white dwarf also varied, rapidly at first and then slow ly. Eclipse measurements in quiescence lead to estimates for the fundamenta l parameters of the binary: q = 0.13 +/- 0.02, M-2 = 0.12 M., M-1 = 0.86 +/ - 0.11 M., and i = 86.degrees8 +/- 1.degrees5. We estimate a distance of 19 0 +/- 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 physi cs.