6 Conclusion and discussion

The topics discussed above concern the reduction technique and our data set from the observational side. Further interpretation, like in-depth determination of the physical status of components, interstellar reddening account, recognition of evolutionary effects etc., exceeds the scope of this paper. We present some conclusions directly related to the observations.

  

Figure 2: $\Delta {\it V}-\Delta({\it B}-{\it V})$ diagram. The two lines delimit an area corresponding to the MS for A0-F0 stars (see text)

• We find that in most cases the secondary components are also early-type A-F stars (60% of $({\it B}-{\it V})_B<0.3$; 93%: $({\it B}-{\it V})_B<0.58$). This means that we deal with mostly physical binaries rather than optical pairs.
  Figure 2 shows $\Delta {\it V}$ versus $\Delta ({\it B}-{\it V})$between the A and B components. The symbol type corresponds to the primary component luminosity class, taken from the WDS (1996) spectral class. Squares are the MS stars, filled triangles - giants and subgiants, open triangles - with unknown luminosity class and "stars" are peculiar objects. Left and right dashed lines represent the inclinations of the MS for A0 and F0 stars, respectively. Many secondaries fall inside or around the area between these two lines; the outliers may represent the optical components, the natural width of the MS, or evolutionary effects. Further comparison of our astrometric data to old measurements will allow us to reject optical pairs, checking the consistency of photometric or spectroscopic parallaxes with hypothetical dynamical ones [12, (Russel & Moore 1940)].
    

  Figure 3: $({\it U}-{\it B})-({\it B}-{\it V})$ diagram: all the primaries (dark heavy crosses) and all but one (WDS 19146-3615B) secondaries (light crosses) are shown. Solid, short- and long-dashed lines represent the dwarfs, giants and supergiants, respectively (taken from [13, Straizys 1977)]

• A large part of our objects have both $({\it U}-{\it B})$ and $({\it B}-{\it V})$ indices of the components measured. For these 75 stars we can construct the two colour diagram (Fig. 3) for primaries and secondaries. One needs to correct the data for interstellar absorption before giving any astrophysical interpretation since our objects are relatively distant ones: the distance modulus m-M varies from $6^{\rm m}$ to $11^{\rm m}$. It can be seen that our stars occupy the thick area between the MS and evolved stars of luminosity classes III-Ia. But, since we do not know so far the reddening factor of our objects we cannot separate the effects of evolution or chemical composition from those of interstellar absorption.
• A lot of faint stars are discovered in a close neighbourhood of our objects; some of them have separations to the primary similar to those in the main pair A+B. Very few of these cases are possible physical multiples. Although trapezium-like configurations are detected among a number of OB-stars (like in the Orion nebula), they must have been already disrupted in substantially older systems of spectral class A (e.g. [4, Eggleton & Kiseleva 1995)]. So, further investigations of our data are required in this field. The existence of low-mass distant companions in A-type multiples is also of great interest.
• The comparison with Hipparcos data reveals the high degree of similarity of our data to those of Hipparcos when available. This proves our astrometric information as good and reliable for investigation of relative motion of A-type double star components.

Acknowledgements

We would like to thank Dr. A. Tokovinin (Sternberg Institute) for discussions of the current work and paper. In the final steps the data from Astronomical Data Center were extensively used.

This work was supported by the OSTC/DWTC of Belgium through a Belgo-Russian Bilateral Cooperation program (N.S.), the project "Pôles d'Attraction Interuniversitaires" P04/05 initiated and financed by the Belgian Federal Scientific Services (DWTC/SSTC) (D.S.); and an IAU traveling grant (N.S.).