Relative weak phase condition for time reversal ($\mathrm{Im}(g_A{g}_V^{\ast })=0$)

If a Fermi interaction contains vector and axial operators with the same time-reversal symmetry component signs, antiunitarity conjugates their coupling constants without exchanging the two operators. After removing a common field phase, invariance requires $g_A/g_V$ real when $g_V\ne 0$. Equivalently $\mathrm{Im}(g_A{g}_V^{\ast })=0$. Fixed intrinsic phases can additionally constrain the common phase; the relative condition is independent of that choice. Adding a Hermitian-conjugate interaction guarantees a Hermitian operator Hamiltonian, but does not guarantee time-reversal symmetry.