  We review the current observational status of the ROSAT sources RX
J1914.4+2456 and RX J0806.3+1527, and the evidence that these are ultra-short
period (<10min) binary systems. We argue that an Intermediate Polar
interpretation can be ruled out, that they are indeed compact binaries with a
degenerate secondary, and that the period seen in the X-ray and optical is the
orbital period. A white dwarf primary is preferred, but a neutron star cannot
be excluded. We examine the capability of the three current double-degenerate
models (Polar, Direct Accretor and Electric Star) to account for the
observational characteristics of these systems. All models have difficulties
with some aspects of the observations, but none can be excluded with confidence
at present. The Electric Star model provides the best description, but the
lifetime of this phase requires further investigation. These ultra-short period
binaries will be strong gravitational wave emitters in the LISA bandpass, and
because of their known source properties will be important early targets for
gravitational wave studies.
