A B S T R A C TThe picture of the young stellar groups in the Canis Major±Puppis±Vela 2158 , l , 2758 section of the Milky Way is studied and updated utilizing uvbyb photometry of intrinsically luminous OB stars. We use all data from the literature to create a sample with 98 per cent completeness to 9.5 mag.The very dense low reddened OB association CMa OB1 is confirmed at a distance of 0.99 (^0.05 s.e.) kpc. Towards Puppis the brightest intrinsically luminous stars do not reveal Pup OB1 and Pup OB2. In the same direction, we separate two small groups, previously related to the association surrounding NGC 2439 at 3.5±4.5 kpc. The first one contains four highly reddened B-type supergiants situated in front of the cluster at 1.03 (^0.14 s.e.) kpc ± much closer to the Sun than has been estimated before. The second one lies north-west from the cluster at 3.2 (^0.23 s.e.) kpc according to our estimate. In the direction to Vela, the bright OB stars are apparently embedded in a dust cloud and spread out between 0.3 and 2.5 kpc, forming clumps over this distance range.In general, the prominent apparent young structures delineated by the brightest intrinsically luminous OB stars in the directions of Canis Major and Vela are some 20± 25 per cent closer to the Sun than has previously been thought. This is in agreement with the Hipparcos results for the Galactic OB associations, and is highly likely to be caused by the overestimation of the spectroscopic distances used in the previous studies.

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The distribution of bright B-type stars in a field with a radius of 58 centred at the Galactic open cluster Cr 121 is studied utilizing Stro Èmgren and Hb photometry. All PPM stars earlier than spectral type A0 are used, revealing a loose nearby structure at a distance of 660± 730 pc, and a compact more distant group, which appears to be a genuine cluster: Cr 121. Based on similar coordinates, distances and positions on the colour±magnitude (CM) and Hertzsprung±Russell (HR) diagrams, 11 photometric cluster members are selected at a mean distance of 1085^41 standard error) pc. The results are discussed in the light of both classical and Hipparcos points of view.

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We present further arguments that the Hipparcos parallaxes for some of the clusters and associations represented in the Hipparcos catalog should be used with caution in the study of the Galactic structure. It has already been shown that the discrepancy between the Hipparcos and ground-based parallaxes for several clusters including the Pleiades, Coma Ber, and NGC 6231 can be resolved by recomputing the Hipparcos astrometric solutions with an improved algorithm diminishing correlated errors in the attitude parameters. Here we present new parallaxes obtained with this algorithm for another group of stars with discrepant data-the galactic cluster Cr 121. The original Hipparcos parallaxes led de Zeeuw et al. to conclude that Cr 121 and the surrounding association of OB stars form a relatively compact and coherent moving group at a distance of Ӎ550-600 pc. Our corrected parallaxes reveal a different spatial distribution of young stellar populace in this area. Both the cluster Cr 121 and the extended OB association are considerably more distant (750-1000 pc), and the latter has a large depth probably extending beyond 1 kpc. Therefore, not only are the recalculated parallaxes in complete agreement with the photometric parallaxes, but the structure of the field they reveal is no longer in discrepancy with that uvbyb found by the photometric method.