Carbon WD mass vs. central mass density. All stars along the dashed curve rotate at P = 29.6 s, as observed for J2056–3014. The solid square marks the least massive WD, whose Kepler period is 29.6 s. For all other stars along the dashed line, the rotation period of 29.6 s is greater than the Kepler period. The most massive WD on the sequence, marked with a solid triangle, has a central density of 9.26 × 10⁹ g cm⁻³. At this density, pycnonuclear reactions set in and microscopic stability ends.