Most stars in the solar neighbourhood are members of multiple systems (e.g. Duquennoy & Mayor 1991; Fischer & Marcy 1992). This raises the question of whether most of these systems were formed as binaries or whether they are the result of later capture processes. After high angular resolution techniques in the near infrared (NIR) had been developed at the beginning of the 1990s this problem became an issue of observational astronomy.
A large number of multiplicity surveys in star-forming regions (SFRs) and clusters has now been done (see Mathieu et al. 2000 and references therein). It is still a matter of debate if different environmental conditions of star formation lead to different degrees of multiplicity. One can however conclude that there is at this time no sample of young stars that shows a significant binary deficit compared to nearby main sequence stars. In some SFRs even a strong binary excess is observed. The consequence of these results is that multiplicity must be already established in very early phases of stellar evolution and that star formation to a large extent has to be considered as formation of multiple stars. Multiplicity has to be taken into account if one asks for stellar properties. If this question is addressed to the systems instead of the components, misleading results may be obtained.
In this paper we will discuss the young binary systems in the nearby SFRs Taurus-Auriga, Upper Scorpius, ChamaeleonI and Lupus that have been detected by Leinert et al. (1993), Ghez et al. (1997a) and Köhler et al. (2000). We present resolved photometry of the components in the NIR spectral bands J, H and K (Sect. 2). Based on these data we discuss the components in a color-color diagram (Sect. 3) and in a color-magnitude diagram (Sect. 4). Using the J-band magnitude as an indicator for the stellar luminosity and published spectral types for the primaries, we place the components pairwise into the Hertzsprung-Russell diagram (HRD, Sect. 5) and derive masses and in particular mass ratios from a comparison with theoretical PMS evolutionary models (Sect. 6). Implications of the results for theoretical concepts of multiple star formation and for binary statistics are discussed in Sect. 7.
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