  We extend recent work that included the effect of pressure forces to derive
the precession rate of eccentric accretion discs in cataclysmic variables to
the case of double degenerate systems. We find that the logical scaling of the
pressure force in such systems results in predictions of unrealistically high
primary masses. Using the prototype AM CVn as a calibrator for the magnitude of
the effect, we find that there is no scaling that applies consistently to all
the systems in the class. We discuss the reasons for the lack of a superhump
period to mass ratio relationship analogous to that known for SU UMa systems
and suggest that this is because these secondaries do not have a single valued
mass-radius relationship. We highlight the unreliability of mass-ratios derived
by applying the SU UMa expression to the AM CVn binaries.
