  The origin of early superhumps, which are double-wave periodic modulations
seen only during the earliest stage of WZ Sge-type outburst, has not been well
understood. Based on recent discovery of two-armed arch-like patterns on
Doppler tomograms in conjunction with early superhumps, we propose a new
interpretation on the origin of early superhumps, following the new
interpretation by Smak (2001) and Ogilvie (2001) of the two-armed pattern seen
in IP Peg. If we consider irradiation of the elevated surface of the accretion
disk caused by vertical tidal deformation, we can consistently explain the
observed features on Doppler tomograms and photometric waves at the same time.
We interpret that a combination of low mass-ratios (q) and low mass-transfer
rates, necessary to give rise to these tidal effects, discriminate WZ Sge-type
stars from other SU UMa-type dwarf novae. Based on recent fluid calculations,
such an effect would be observable in higher q systems. We interpret that RZ
Leo is an example of such objects.
