Building the Dwarf Nova NS Per in the Young Open Cluster NGC 1528 Requires Inefficient Common-envelope Ejection

Abstract Common-envelope (CE) ejection determines which close binaries survive to become cataclysmic variables (CVs), but its efficiency, α CE , cannot be measured cleanly in field systems, where the progenitor mass, metallicity, and elapsed time are known only through the models being tested. The dwarf nova NS Per avoids this circularity. It is an astrometric member of the open cluster NGC 1528, whose catalog age of t cl = 209 Myr is independent of the binary, and its accreting state guarantees that the post-CE inspiral is complete. From 33 absorption-line radial velocities of the donor obtained with the MDM 2.4 m telescope, we derive P orb = 6.29500 ± 0.00050 hr, and we compare this period with synthetic binary populations computed over a range of ejection efficiencies. In the population that reproduces NS Per, the white dwarf forms after ∼95 Myr, and only ∼19 Myr of detached inspiral separate envelope ejection from the onset of mass transfer, so the observed period still records the orbit at ejection. Within this population-synthesis framework, the cluster age implies α CE = 0.26 (0.20–0.33, 68% credible; 90% upper limit 0.35) and excludes α CE ≥ 0.70: efficient ejection leaves the white dwarf less massive than its donor, which precludes the stable mass transfer observed in a dwarf nova. The posterior predicts M WD = 0.94 (0.85–1.02) M ⊙ . NS Per is the first accreting binary whose CE efficiency is calibrated against an externally supplied age, and the inefficient ejection it requires agrees with the low α CE favored by post-CE binaries, double white dwarfs, and the white-dwarf mass distribution of CVs.