Northern late-type nitrogen-rich WR (WNL) spectra

By using Monte Carlo sampling and a Bayesian framework, we computed the three-dimensional likelihood and one-dimensional posteriors for the upper period cut-off (logP_max_^WNL^), power-law index ({PI}^WNL^), and intrinsic binary fraction (f_int_^WNL^). Adopting a threshold C of 50km/s, we derived f_obs_^WNL^=0.36+/-0.15. Our Bayesian analysis produces f_int_^WNL^=0.42_-0.17_^+0.15^, {PI}^WNL^=-0.70_-1.02_^+0.73^ and logP_max_^WNL^=4.90_-3.40_^+0.09^ for the parent WNL population.

The combined analysis of the Galactic WN population results in f_int_^WN^=0.52_-0.12_^+0.14^, {PI}^WN^=-0.99_-0.50_^+0.57^ and logP_max_^WN^=4.99+_1.11_^+0.00^. The observed period distribution of Galactic WN and WC binaries from the literature is in agreement with what is found. The period distribution of Galactic WN binaries peaks at P~1-10d and that of the WC population at P~5000d.

This shift cannot be reconciled by orbital evolution due to mass loss or mass transfer. At long periods, the evolutionary sequence O (->LBV) -> WN -> WC seems feasible. The high frequency of short-period WN binaries compared to WC binaries suggests that they either tend to merge or that the WN components in these binaries rarely evolve into WC stars in the Galaxy.