Mixing of CNO-cycled matter in pulsationally and magnetically active massive stars

We report on the abundances of helium, carbon, nitrogen and oxygen in a larger sample of Galactic massive stars of ~7-20 M_solar near the main sequence, composed of apparently normal objects, pulsators of β-Cephei- and SPB-type, and magnetic stars. High-quality spectra are homogeneously analysed using sophisticated non-LTE line-formation and comprehensive analysis strategies. All the stars follow a previously established tight trend in the N/C-N/O ratio and show normal helium abundances, tracing the nuclear path of the CNO-cycles quantitatively. A correlation of the strength of the mixing signature with the presence of magnetic fields is found. In conjunction with low rotation velocities this implies that magnetic breaking is highly efficient for the spin-down of some massive stars. We suggest several objects for follow-up spectropolarimetry, as the mixing signature indicates a possible magnetic nature of these stars.