  Using the 2dF multi-fibre instrument on the AAT, moderate resolution spectra
have been obtained for a large sample of stars on the main sequence and at the
turnoff in the unusual globular cluster omega Centauri. We investigate the
behaviour of CH, CN and SrII line strength indices as a function of overall
abundance for the main sequence sample. A number of stars do not follow the
relations defined by the majority. These anomalous objects can be categorised
into (at least) three types. (1) Carbon enhanced stars, which represent about
5% of the sample, and which are found at all metallicities. Spectrum synthesis
calculations show that the atmospheres of these stars are typically enhanced in
C by factors of between 3 and 10. (2) Nitrogen enhanced stars, revealed for
[Fe/H] > -1.3 by strong CN indices, which make up ~40% of the cluster main
sequence population above this metallicity. These stars are enhanced in N by
factors of up to 100. Our data, however, provide no constraints of their
relative numbers at lower [Fe/H]. (3) Stars with enhancements of the s-process
element Sr by factors of 30 to 60. The possible origins for these abundance
anomalies are discussed.
