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Published online by Cambridge University Press: 12 April 2016
It is shown that the assumption of a black dwarf secondary in Z Cha leads to a number of contradictions with well established theoretical and observational facts. In particular the model predicts the radial velocity K1 to be much smaller than is observed, the radius of the accretion disk to be smaller than is physically possible and a white dwarf mass which is inconsistent with the white dwarf's radius derived from eclipse analysis. Using the same arguments as for Z Cha it is also possible to exclude a black dwarf secondary in the similar systems OY Car and HT Cas.