Spectroscopy of IY Ursae Majoris during superoutburst

We report time-resolved spectroscopic observations of the recently discover ed dwarf nova, IY Ursae Majoris, over 2 days during its 2000 January supero utburst. The spectra show prominent, broad, and double-peaked emission line s of H alpha, H beta, H gamma, H delta, and He II and weaker He I lines. Th e radial velocity curve of the peaks of H alpha and H beta has an amplitude of 93 +/- 7 km s(-1) and a phase offset of 0.12 +/- 0.03. From clear evide nce for a precessing disk, we obtain a solution to an eccentric outer disk consistent with theoretical works, which demonstrates the validity of the r elation between superhumps and tidal effects. The gamma velocity of the bin ary is derived as -4 +/- 32 km s(-1) according to the solution. The inner p art of the disk is also eccentric as evidenced by asymmetric wings in the l ines. Therefore, the whole disk is eccentric. Finally, we find that a simpl e model can explain all asymmetric features of the emission lines.