 Unit index of a CM-field

For a CM-field, put $c$ for conjugation and $E_K={\mathcal{O}}_K^{\times }$. The map $u\mapsto u/c(u)$ takes values in the cyclic group ${\mu }_K$ of roots of unity: all conjugates have absolute value one, and the Kronecker theorem on algebraic integers in the unit disk applies. Its kernel is $E_{K^{+}}$, while its image on ${\mu }_K$ is ${\mu }_K^2$. It therefore embeds $E_K/(E_{K^{+}}{\mu }_K)$ into ${\mu }_K/{\mu }_K^2$, a group of order two. This proves the index bound without assuming that every unit is a real unit times a root of unity.