1STRUCTURE AND EVOLUTION OF NEARBY OB ASSOCIATIONS

We present the rst results of a comprehensive cen-sus of the stellar content of the nearby OB associa-tions based on Hipparcos positions, proper motions and parallaxes for 12842 candidate member stars dis-tributed over 21 elds on the sky. We use a new method to identify moving groups in these elds (see de Bruijne et al., these proceedings). Previously, as-trometric membership in nearly all the nearby OB associations was known only for stars with spectral types earlier than B5. The Hipparcos measurements now allow us to identify members down to late F. This census provides a rm basis for studies of galac-tic and extragalactic star forming regions. Key words: OB associations; HR diagram; luminos-ity calibration; stars: early-typ