WZ Sagittae star
A WZ Sagittae star is a variety of SU Ursae Majoris star (a type of dwarf nova) in which the interval between super-outbursts is unusually long, measured in decades, while normal outbursts are few and far between. WZ Sagittae itself has had super-outbursts at intervals of about 33, 32, and 23 years, and has never been seen to undergo a normal outburst. Following its first recorded brightening, in 1913, WZ Sge was classified as a classical nova and named Nova Sagittae 1913. When it erupted again, in 1946, it was recategorized as a recurrent nova. Further observations revealed an orbital period (of the white dwarf primary and its companion) of 81 minutes 38 seconds – -much shorter that of any known recurrent nova-and a spectrum, lacking broad emission bands, that is more characteristic of a dwarf nova. Observations of the 1978 outburst also revealed super-humps in WZ Sge's light curve, which are the defining characteristics of SU UMa type dwarf novae; thus WZ Sge is now considered the prototype for a subset of the SU UMa class. Other WZ Sge stars include AL Com and EG Cnc, which have super-outburst intervals of approximately 20 years.
The approximately 30-year supercycle length that WZ Sge displays is the most inactive group of the SU UMa type stars. The factor determining the different timescales appears to be mass-transfer rate. WZ Sge stars have a very low mass-transfer rate, perhaps only 1012 kilograms per second. Given the slow rate of mass-transfer, it would then take decades to accumulate enough material for a super-outburst. The puzzle of these stars, however, is why they show few or no normal outbursts during this interval. Even with a low mass-transfer rate, material should accumulate, drifting viscously into the inner disk, and trigger an outburst. One suggestion for why this does not occur is that the disk viscosity is very low. The material would then remain in the outer disk, where much more can be stored before an outburst is triggered. The problem with this idea, however, is to explain the extremely low viscosity level. Another possible explanation involves the removal of the inner disc, to prevent outbursts starting there. This could occur through siphons or because of a magnetic field on the white dwarf.