White Dwarfs in Ultrashort Binary Systems
  This contribution summarizes, as of early 2008, the observational and
theoretical understanding of the physics, and emission properties of two Double
Degenerate Binaries with the shortest orbital period known to date. In
particular, the Unipolar Inductor Model and its coupling to GW emission have
been introduced to explain a number of puzzling features that these two sources
have in common and that are thought difficult to reconcile with models of mass
transfer in such systems. Emphasis was put on the relevant new physical
features that characterize the model. The role of spin-orbit coupling through
the Lorentz torque and the role of GW emission in keeping the electric
interaction active at all times has been thoroughly discussed in all their
implications. Application of the model to both HM Cnc and V407 Vul accounts in
a natural way for their main observational properties. Constraints on physical
parameters are derived in order for the model to work, and can be verified by
future observations.
