  Seesaw models with a slightly broken lepton number symmetry can explain small
neutrino masses, and allow for low-scale leptogenesis. We make a thorough
analysis of leptogenesis within the simplest model with two right-handed (RH)
neutrinos (or with N_3 decoupled). We obtain a semi-analytical formula for the
final asymmetry in both supersymmetric and non-supersymmetric cases with a
simple dependence on each parameter. The low-energy parameters factorize from
the high-energy ones, and the high-energy phase must be non-zero. The role of
the PMNS phases is carefully studied. Moreover, we find that the breaking
parameter in the Yukawa coupling matrix must be relatively large, \epsilon_h >
10^{-3} for normal and 10^{-2} for inverted hierarchy. Therefore, leptogenesis
in our simple model is incompatible with RH neutrino signals at future
colliders or sizable lepton-flavor violation. The other breaking parameter,
\epsilon_M, which appears in the RH neutrino mass matrix, can be much smaller,
and actually needs to be so in order to have low-scale leptogenesis.
