¹ Observatoire de Genève, CH-1290 Sauverny, Switzerland
² UCO/Lick Observatory, University of California, Santa Cruz CA 95064, USA
³ Department of Astronomy, University of Washington, Box 351580, Seattle WA 98195, USA
⁴ Institut d'Astronomie, UniL, CH-1290 Chavannes-des-Bois, Switzerland

The number ratios Be/(B+Be) of Be to B-type stars in young, well studied clusters of the Galaxy, the LMC and SMC are examined. In order to disentangle age and metallicity effects we choose clusters in the same age interval and for which reliable photometric and spectroscopic data are available. Number counts are made for various magnitude intervals, and the results are found to be stable with respect to this choice. In the magnitude interval M_V = -5 to -1.4 (i.e.  O9 to B3) we obtained a ratio Be/(B+Be) = 0.11, 0.19, 0.23, 0.39 for 21 clusters located in the interior of the Galaxy, the exterior of the Galaxy, the LMC and the SMC, respectively.

Various hypotheses for these differences are examined. An interesting possibility is that the average rotation is faster at low metallicities as a result of star formation processes. The much higher relative N-enrichment found by Venn (1999) in A-type supergiants of the SMC, compared to galactic supergiants, also strongly supports the presence of more rotational mixing at low metallicities. We discuss whether high rotational mixing may be the source of primary nitrogen in the early chemical evolution of galaxies.

1999, A&A, 346, 459