7 Comparison with colour-magnitude diagrams

  As expected, there is a large scatter in the photographic (B-V) colours compared to those of the CCD photometry of Trullols & Jordi (1997). Therefore, the photographic V-(B-V) CMD can hardly be used as an additional tool in the membership determination and is not shown here. Obviously, the fluctuation of extinction in B (and V) over the one square degree field and the variation of the sky background connected with photographic effects make the photographic V-(B-V) CMD nearly useless. Note that the standard stars used for our photographic photometry are concentrated in the very central part of the field (see Fig. 1).

The situation becomes slightly better with the photographic V-(V-R) CMD shown in Fig. 13 in comparison with the CCD photometry of Trullols & Jordi (1997). Here the scatter in the photographic (V-R) colours is not as large in comparison to the standard stars CCD photometry, unless small systematic deviations comparable to the errors of the photographic photometry can be seen. A more detailed analysis of these effects will be subject of a forthcoming paper (Belikov et al., in preparation).

  

Figure 13: Colour-Magnitude diagram V - (V-R) for all 1083 objects (dots) from our sample of 1431 stars with magnitudes and colours obtained from the POSS Quick-Survey V plate and the average of two red POSS plates after the POSS1 E plate magnitudes were transformed to the POSS2 R plate system by Eq. (3). For comparison, the stars from Trullols & Jordi (1997) are plotted with their CCD magnitudes as open triangles (members according to their classification) and open circles (nonmembers). Five out of 25 stars from Trullols & Jordi (1997) which are in the proper motion sample are clear nonmembers (drawn as +) according to our proper motions ($P_{\rm cl} < 5$%) but were classified as members by Trullols & Jordi (1997). On the other hand two of these 25 standard stars (drawn as $\times$) classified as nonmembers by Trullols & Jordi (1997) may be members according to our proper motion data ($P_{\rm cl} \gt 50$%)