... objects

Alexander von Humboldt Fellow at the Max-Planck-Institut für Astronomie

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Appendices are only available in electronic form at http://www.aanda.org

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...$\sigma$ Orionis

Jeffries et al. (2006) and Caballero (2007) have discussed the spatial overlapping of different young populations of the Ori OB 1b Association in the direction of $\sigma$ Orionis.

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