  We calculate, using unitarity, a lower bound on the branching ratio
$\chi\chi\to \gamma\gamma$ and $\chi\chi\to \gamma Z$, where $\chi$ is any halo
dark matter particle that has $W^+W^-$ as one of the major annihilation modes.
Examples of such particles are supersymmetric particles with a dominant
Higgsino component, or heavy triplet neutrinos. A substantial branching ratio
is found for the $\gamma\gamma$ and $\gamma Z$ modes. We estimate the strength
of the monoenergetic $\gamma$ ray lines that result from such annihilations in
the Galactic or LMC halos. (Latex file; 2 compressed uuencoded postscript
figures available by anonymous ftp from vanosf.physto.se in file
pub/figures/lines.uu)
