Spectroscopic Study of SU UMa-type Dwarf Nova YZ Cnc during its 2002 Superoutburst

We report time-resolved spectroscopic observations of the SU Ursae Majoris dwarf nova, YZ Cnc, for 2 nights over 11 hrs during its 2002 January superoutburst. The spectra only show absorption-line profiles in the first day. But the lines display blue and red troughs, with ``W'' profiles in the second day. The radial velocity curve of the absorption troughs and emission peaks of H$\beta$ has an amplitude of $49\pm10$ km s$^{-1}$ and a phase offset of $-0.07\pm0.04$, which are very similar to those measured in quiescence; however, the $\gamma$ velocity deviates strongly from the systemic velocity measured in quiescence, showing variation of the order of $\pm$60 km s$^{-1}$. And large shifts of $\sim$70 km s$^{-1}$ and $\sim$0.09, for the orbital-averaged velocity and phase respectively, are also found in our observations. All these phenomena can be well explained with a precession of an eccentric disk and we conclude that these phenomena are the characteristic products of an eccentric accretion disk.