Constraints on doubly charged Higgs interactions at linear collider
  Production of a single doubly charged Higgs boson $Delta^{--}$ in polarized $e^+e^-$ and $e^+\gamma$ collision modes of the linear collider have been investigated. The mass range of $Delta^{--}$ to be probed extends up to the collision energy. The diagonal lepton number violating Yukawa coupling $h_{ee}$ will be tested at least three orders of magnitude more strictly than in present experiments. 
G. Barenboim, K. Huitu, J. Maalampi, M. Raidal