  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 (1979). 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".

Astrophysics