Two nitrogen rich main sequence B-stars in the Small Magellanic Cloud cluster, NGC 330.

High resolution spectra of two narrow lined main-sequence B-type stars in the SMC cluster NGC 330 are analysed using LTE methods to obtain their chemical compositions relative to SMC and galactic field B-stars. It is found that these stars, designated A01 and B04, have parameters ({T_eff_, logg}) of {26000, 3.8} and {23000, 3.6} respectively and, surprisingly, nitrogen abundances approximately 0.8dex higher than other SMC main-sequence B-stars. There is also marginal evidence for moderate deficiencies of both oxygen and silicon consistent with the previously reported low metallicity of this cluster. Since supergiants in the cluster do not exhibit such a large nitrogen enrichment it is argued that A01 and B04 have had their surface chemical compositions modified either as the result of binary mass transfer or due to rotationally induced turbulent diffusion. For both scenarios the absence of a complementary carbon deficiency is surprising. These findings, combined with previous observations of the most luminous main-sequence stars in the cluster, imply that the upper main-sequence of the cluster is difficult to understand in terms of standard stellar evolution models.