Accurate Quantitative Spectroscopy of OB Stars: C and N abundances near the Main Sequence

We present a state-of-the-art analysis technique able to simultaneously reproduce the entire H and He spectra of OB-type stars in the visual and the near-IR and to derive highly accurate metal abundances (so far C and N). The spectrum synthesis relies on a hybrid non-LTE approach involving our most recent model atoms. Accurate atmospheric parameters, with reduced systematic errors, are derived spectroscopically (from Stark-broadened H lines and ionization equilibria of He I/II and C II-IV) for a sample of randomly distributed stars in the solar vicinity. Highly consistent abundances are found in contrast to previous reports indicating broad scatter and large uncertainties. The improvements result from avoidance of systematic errors in the parameter determination, which may be larger than expected in previous work, and a critical evaluation of atomic data for the model atom construction.