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

We report on time-resolved spectroscopic observations of the SU Ursae Majoris dwarf nova, YZ Cnc for over 11 h on two nights during its 2002 January superoutburst. The spectra on the first day only showed absorption-line profiles, while on the second day the lines showed ``W" profiles with blue and red troughs. The radial velocity curve of the absorption troughs and emission peaks of H β has an amplitude of 49±10 km s −1 and a phase offset of −0.07±0.04 , which are very similar to those measured in quiescence. However, the γ velocity deviates strongly from the systemic velocity measured in quiescence, by some ± 60 km s −1 . Large shifts of ~ 70 km s −1 in the orbital-averaged velocity and ~ 0.09 in the phase are also found in our observations. All these features can be well explained by a precessing, eccentric disk.