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Context. AM CVn binaries are potential progenitors of thermonuclearsupernovae and strong sources of persistent gravitational wave radiation. For along time, it has been believed that these systems cannot descend fromcataclysmic variables (CVs), at least not in large numbers, because the initialconditions need to be fine-tuned and, even worse, the resulting surfacehydrogen abundance would be high enough to be detected which contradicts adefining feature of AM CVn binaries.  Aims. Here we show that both claimed weaknesses of the CV formation channelfor AM CVn binaries are model-dependent and rely on poorly constrainedassumptions for magnetic braking.  Methods. We performed binary evolution simulations with the MESA code fordifferent combinations of post-common-envelope white dwarf and companion massesas well as orbital periods assuming the CARB model for strong magnetic braking.  Results. We found that AM CVn binaries with extremely-low surface hydrogenabundances are one natural outcome of CV evolution if the donor star hasdeveloped a non-negligible helium core prior to the onset of mass transfer. Inthis case, after hydrogen envelope exhaustion during CV evolution, the donorbecomes degenerate and its surface hydrogen abundance substantially drops andbecomes undetectable. Our simulations also show that the CV formation channelis able to explain the observed AM CVn binaries with very low mass and bloateddonor stars (Gaia14aae and ZTF J1637+49).  Conclusions. CVs with evolved donors are likely the progenitors of at least afraction of AM CVn binaries.

Reversing the verdict: Cataclysmic variables could be the dominant progenitors of AM CVn binaries after all