EZ Lyn: A Confirmed Period-Bouncer Cataclysmic Variable Below the Period Minimum

We model the short-period cataclysmic variable EZ Lyn with MESA binary evolution and infer its present-day parameters through a staged statistical search. First, we compute a coarse grid of tracks in (M1,0,P0) at fixed M2,0 and rank snapshots by a profile likelihood. We then resample the neighbourhood of the minimum to build a refined Δχ2 surface. Finally, we sample this surface with an affine-invariant MCMC to obtain posteriors, using a likelihood that treats the one-sided constraint on the donor temperature and the ambiguity of component roles in the binary output. The best-fit snapshot reproduces the observables and identifies EZ Lyn as a period bouncer with a substellar donor. We infer MWD=0.850±0.019M⊙, M2=0.0483±0.0137M⊙, RWD=0.0092±0.0001R⊙, R2=0.099±0.005R⊙, TWD=11,500±20K, and T2=1600±50K. The instantaneous mass-transfer rate at the best-fit snapshot is M˙=3.66×10−11M⊙yr−1, consistent with the secular range implied by the white-dwarf temperature. Independent checks from the Roche mean-density relation, surface gravities, and the semi-empirical donor sequence support the solution. In population context, EZ Lyn lies in the period-minimum spike and on the low-mass tail of the donor mass–period plane. The classification is robust to modest displacements along the shallow Δχ2 valley. We release inlists, tracks, and analysis scripts for reproducibility.