Colour-magnitude diagram.

 
 

Figure 3: Colour-Magnitude diagram for 337 stars in NGC 6789. Circles correspond to stars measured in the central galaxy region (C), filled diamonds indicate main body (E) ones, dots represent probable foreground.

The CM diagrams of regions C,E and F (see Fig.1^b) are shown in Fig. 3, where stars from region C are denoted by open circles, ones from the E are filled diamonds and stars from the region F are shown by dots.

The [(V-I),I] CM diagram of the NGC 6789 shows characteristics typical of the CM diagrams of other dwarf galaxies, with evidence for both old and young populations. The main feature of the central part stars of NGC 6789 CM diagram is the clump that extends between and , which is composed of the RGB and the asymptotic giant branch (AGB) locus of old and intermediate-age stars and by intermediate-age blue loops. Unfortunately, we have not so many stars to say whether this clump is solid, or it is split into several small clumps in such a way that it is difficult to decide which of them actually marks the TRGB. We will discuss this in detail below in §3.2. There is also a considerable population of blue stars ((V-I)<0.7) in the CM diagram of the central region. It is remarkable that these blue stars are entirely absent at the CM diagram of the outer part of the galaxy. The presence of these stars in the central region, as well as the candidate HII regions in H image, supports the idea that NGC 6789 is a BCD galaxy rather than a dE. Most of the bright stars (I<20.5) are likely foreground stars. According to our our spectral long slit observations the brightest foreground star near the center of the galaxy belongs to the F spectral type.

The diagram shown in Fig. 3 have not been corrected for interstellar extinction. However, the extinction must be considered in the forthcoming calculation of metallicity and distance. The galactic coordinates of NGC 6789 are , and it lies in a region of moderate gradient and near the isoline corresponding to in the extinction maps of Burstein & Heiles (1984). Using the extinction law by Cardelli et al., ([1989]) with R_V=3.3, the extinction is: A_B=0.32, A_V=0.24, A_R=0.21, and A_I=0.15.