Measured orbital periods and donor star masses and radii for mass-transferring UCBs in the catalogue. In the left panels, we plot the observed properties: orbital period and donor mass. In the right panels, we plot donor mass and radius. Uncertainties in the right panels are diagonal because of the strong constraint on donor density that arises from the orbital period. The two systems plotted with red squares are the brown dwarf donor SDSS J1507+5230 and the 30 min supernova progenitor candidate ATLAS J1138-5139. We also plot several evolutionary tracks from different scenarios, as discussed in the text: the upper panels show evolutionary models of the white dwarf and helium star donor tracks, while the lower panels show evolutionary models from the evolved CV scenario. The systems without hydrogen (AMCVn-type binaries) follow a sequence from short to long orbital periods, while He CVs with hydrogen follow a different sequence from low to high masses. In general, any formation scenario can explain at least some fraction of the known systems, and no single formation scenario can explain every system.