5 Discussion

What was mentioned in the introduction and in previous papers, namely that photometric data are severely lacking for visual binaries, was confirmed when we consulted the SIMBAD data base of the CDS (Strasbourg, France): hardly any UBV photometry was found for our sample. In addition, there was not any kind of information for five of our targets; these objects are identified by the truncated 2000 coordinates given in the WDS. Notice also that 18 of our targets have no HD identifier.

We could not find any correlation between the measured $\Delta V$ and $\Delta (B{-}V)$, nor between $\Delta V$ and $\Delta (U{-}B)$;the two colour diagram of Fig. 4, however, shows a strong linear correlation between $\Delta (B{-}V)$ and $\Delta (U{-}B)$.Components of sixteen binaries have practically the same (B-V) and (U-B) colours, which means the same $T_{\rm eff}$ as well. In particular the binary with primary DM $-26^\circ$ 943 has components that may be considered to be identical stars at identical distances.

The astrometric results show again a very satisfactory internal accuracy, confirming the power of the present CCD observational techniques for the collection of these data.

Table 3 shows that the angular separations presented here are identical with the ones of HIPPARCOS for the common targets.

For these targets, HIPPARCOS gives position angle results rounded to one degree.

  

Table 3: Comparison between HIPPARCOS and our angular separations (in arcseconds)

Our position angles are obviously not rounded and thus expected to be one order of magnitude more accurate. In any case, the position angles of both sources are identical at the statistical level of the HIPPARCOS results (one degree).

Unfortunately, HIPPARCOS gives the magnitude difference between components in the $\Delta Hp$ system of the satellite. These magnitude differences are not comparable with our $\Delta V$ values. They usually differ a few hundreds of magnitudes, but differences up to a few tenths of magnitudes can occur.

Acknowledgements

This research was mainly carried out under contracts with SSTC/DWTC (Services du Premier Ministre Belge - Affaires Scientifiques, Techniques et Culturelles: "Service Centres and Research Networks'' SC/005 and "Pôles d' Attraction Interuniversitaires'' P4/05). This research has made use of the Simbad database, operated at CDS, Strasbourg, France. We thank Dr. E. Oblak for his useful remarks.