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This website and the Z Cam List created and maintained by Mike Simonsen. 

Introduction to Z Camelopardalis type dwarf novae (UGZ)
Mike Simonsen (2009)

UGZs are defined in the General Catalog of Variable Stars as dwarf novae that “show cyclic outbursts, differing from UGSS variables by the fact that sometimes after an outburst they do not return to the original brightness, but during several cycles retain a magnitude between maximum and minimum. The values of cycles are from 10 to 40 days, while light amplitudes are from 2 to 5 magnitudes in V.”

So it’s all about the standstills, those episodes where the star gets stuck at a mid-point between maximum and minimum. If it doesn’t exhibit standstills it isn’t a Z Cam star.

Typical standstill of Z Camelopardalis

So UGZ can be classified by their light curves alone. Orbital period is not a factor in classification, even though they all tend to be on the long side of the period gap, 3 hours to 10 hours orbital period.

There is no strong agreement between the various CV catalogs as to which few dozen or so stars are actually Z Cam type systems. There are a handful of bright objects that have been densely covered by amateurs throughout their range that are obviously UGZ from their light curves. They show the typical Z Cam-like standstills, have short outburst cycles and amplitudes around 3 or 4 magnitudes.

There are also some bright systems listed as UGZ, like AB Draconis, that have the short cycle and small amplitude, don’t show obvious standstills, and yet it seems everyone agrees they are UGZ.

AB Draconis- where are the standstills?

There are many more CVs that have some of the characteristics of UGZ, but it is not at all apparent from the existing data that they show standstill behavior because the range at which you would expect to see this, somewhere mid-point between maximum and minimum brightness, is too faint for visual observes to have accumulated useful data over the years. All we really know from the data is the average outburst cycle and approximate amplitude. There is no detail in the middle where the real story lies.

Depending on which catalog you use, there are only 30 to 40 Z Cam dwarf novae. If any significant percentage of the number of Z Cams eventually proves not to be Z Cam, the remaining few represent a fairly rare and unique class of stars worthy of further investigation.

To aid in this investigation we have created The Z Cam List, a list of all the known, suspected and previously misidentified Z Cam stars in the literature. The AAVSO Cataclysmic Variable Section has also initiated a long-term campaign to study these stars, The Z CamPaign.

Our goal is to bring together the most complete, up to date, accurate information on these unique stars into one place, The Z Cam List.