V3101 Cyg: A Cataclysmic Variable Born with a Brown Dwarf Donor

Cataclysmic variables descending from detached brown dwarf plus white dwarf binaries have long been predicted but none have been found so far. We report here on a system whose origin is best explained by said scenario: V3101 Cyg. We measured a white dwarf mass of Mwd = 0.47 ± 0.01 M⊙ from the analysis of far-ultraviolet HST spectroscopy which, combined with the published mass ratio of q = Mbd/Mwd = 0.088, implies a donor star mass of Mbd = 0.041 M⊙, well below the hydrogen burning limit. This makes V3101 Cyg the only known object composed of both a low-mass white dwarf and a substellar companion. We modelled the past evolution of this system using MESA, and find that the standard cataclysmic variable evolutionary channel fails to reproduce the donor’s estimated mass and radius, as well as the low quiescent temperature of both components – measured from archival photometry taken before the system experienced a ≃9.5 mag outburst in 2019. In contrast, the observed characteristics of V3101 Cyg can be well reproduced in MESA by progenitors born with a brown dwarf companion. Based on our results, V3101 Cyg represents the first unambiguous observational confirmation of the brown dwarf plus white dwarf evolutionary channel, and provides a first opportunity to quantitatively test the predictions of close binary population models