  We investigate the effects of including strong charge-parity (CP) violating
effects through axion fields in the microscopic equation of state of massive
hybrid neutron stars. We assume that their cores contain deconfined quark
matter and include the effects of axions via an effective 't Hooft determinant
interaction. The hadronic crusts are described using different approaches in
order to make our results more general. We find that the presence of axions
stabilizes massive hybrid neutron stars against gravitational collapse by
weakening the deconfinement phase transition and bringing it to lower
densities. This enables to reproduce hybrid neutron stars in agreement with
modern astrophysical constraints.
