1999A&A...345..597P

Abundances for the light elements of λ Bootis stars are a main key to understand the astrophysical processes behind the so-called λ Bootis phenomenon. These stars are characterized by a typical abundance pattern (strong underabundances of the Fe-peak elements whereas the light elements have apparently solar abundances) which is still based mainly on LTE-calculations. Therefore we started an investigation to derive accurate abundances of the light elements (C, N, O and S). For this purpose a new oxygen model atom was implemented in the Kiel non-LTE code. High resolution and high signal-to-noise spectra were used. For each element only a single wavelength region with lines of the specific element was selected and observed in order to avoid contamination from other elements. In the first paper we present abundances for carbon and oxygen of a statistically significant number of well established λ Bootis stars. The second paper will deal with nitrogen and sulphur. The most important result is that on average carbon is less abundant than oxygen but still both elements are significant more abundant than the Fe-peak elements. Furthermore the anticorrelation of carbon and oxygen with the silicon abundance is proven, which strongly supports the accretion/diffusion theory.