  Shortly after the discovery of the binary system PSR J0737-3039 we predicted
that it has a small proper motion and suggested that this implies that the
progenitor mass of the younger pulsar (B) must be around 1.45M_sun. This is in
contradiction to standard evolutionary scenarios that suggest a He-star
progenitor with M >2.1-2.3M_sun and in sharp contrast to a velocity of ~100
km/s predicted by these models. It requires a new previously unseen type of
gravitational collapse. Our (generally ignored) prediction of a low peculiar
motion was confirmed later by pulsar timing observations that found v_\bot < 30
km/s. Recently Willems et al. put forward an elaborate simulation suggesting
that in spite of these observations, the most likely natal kick is large
(50-170 km/s) and the corresponding most likely progenitor mass is large as
well. We refute this claim and show that Willems et al. implicitly assume the
result that they obtain.

Astrophysics