Early humps in WZ Sge stars

Photometric humps in outburst that are locked with the binary orbital period have been observed exclusively in the early phase of outbursts of WZ Sge stars. It is suggested that this ``early hump'' phenomenon is the manifestation of the tidal 2:1 resonance in accretion disks of binary systems with extremely low mass ratios. The ``early humps'' can be understood by the two-armed spiral pattern of tidal dissipation generated by the 2:1 resonance, first discussed by Lin & Papaloizou (\cite{Lin79}). The tidal removal of angular momentum from the disk during outbursts of dwarf novae, an important feature, is discussed in the context of the disk instability model. The ordering of tidal truncation radius, the 3:1 and 2:1 resonance radius in systems of different mass ratio naturally leads to a classification of dwarf nova systems in three groups according to their mass ratio. The WZ Sge stars are those systems which have the lowest mass ratios and are therefore characterized by ``early humps''.