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In realistic unified models involving an $SO(10)$-like pattern of Dirac andheavy right-handed (RH) neutrino masses, the lightest right-handed neutrino$N_1$ is too light to yield successful thermal leptogenesis, barring highlyfine tuned solutions, while the second heaviest right-handed neutrino $N_2$ istypically in the correct mass range. We show that flavour coupling effects inthe Boltzmann equations may be crucial to the success of such $N_2$ dominatedleptogenesis, by helping to ensure that the flavour asymmetries produced at the$N_2$ scale survive $N_1$ washout. To illustrate these effects we focus on$N_2$ dominated leptogenesis in an existing model, the A to Z of flavour withPati-Salam, where the neutrino Dirac mass matrix may be equal to an up-typequark mass matrix and has a particular constrained structure. The numericalresults, supported by analytical insight, show that in order to achievesuccessful $N_2$ leptogenesis, consistent with neutrino phenomenology, requiresa "flavour swap scenario" together with a less hierarchical pattern of RHneutrino masses than naively expected, at the expense of some mild fine-tuning.These results may be relevant for other $SO(10)$-like unified models where$N_2$ leptogenesis is necessary.

Successful $N_2$ leptogenesis with flavour coupling effects in realistic unified models