= Solution
The leptons consist of
$$
L_{iL}=(\nu_{iL},e_{iL})^{\mathsf T}\sim(1,2)_{-1/2},
\qquad
e_{iR}\sim(1,1)_{-1},
$$
with charged-lepton Yukawa coupling $-\overline L_LY_eHe_R+\mathrm{h.c.}$
Adding gauge-singlet right-handed neutrinos $\nu_R\sim(1,1)_0$ permits the Dirac Yukawa coupling
$$
-\overline L_LY_\nu\widetilde H\nu_R+\mathrm{h.c.},
$$
which gives $M_D=vY_\nu/\sqrt2$. Because $\nu_R$ carries no Standard Model gauge charge, a <Majorana mass term> $\nu_R^{\mathsf T}CM_R\nu_R/2$ is also allowed. For $M_R\gg M_D$, the <seesaw mechanism> produces light neutrino masses of order $M_DM_R^{-1}M_D^{\mathsf T}$.
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