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 \pm 10 km s ^ { -1 } and a phase offset of -0.07 \pm 0.04 , 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 .