and 
, which were used to
point main body stars (C), pereferical (E) and foreground stars (F).
Recent years observations have shown that the division of dwarf galaxies into irregulars (dIrs) and ellipticals (dEs + dSphs) is often very symbolic and quite ambiguous. Dwarf galaxies vary widely in shape and properties and call to a revision of our current understanding of their nature and evolution.
NGC 6789 belongs to the group of blue compact dwarf (BCD) galaxies, which have an irregular patchy blue central part with large HII regions and bright blue stars, and shows no prominent nucleus, like dIrs, while its outer regions have smooth spheroidical shape and old red stellar population, as dwarf ellipticals. According to the classification scheme of Loose & Thuan ( 1985 ), NGC 6789 may be classified as "iE" type, comprising of BCD, which combines irregular inner isophotes with accurately elliptical outer isophotes. The intensive star formation observed in the central region of NGC 6789 contrasts with the absence of the significant young population in the outer regions and suggests a two-component (core-halo or disk-halo) structure of the galaxy. This seems to be common not only in large spirals but also in dwarfs (see for example the cases of WLM; Minniti & Zijstra 1996 , and Antlia; Aparicio et al. 1998 ). Nearby galaxies GR 8 (Tikhonov & Drozdovsky, 1998b ), NGC 4163 (Tikhonov & Karachentsev 1998a ), NGC 1705 (Meurer et al., 1992 ), NGC 2915 (Meurer et al., 1996 ), and UGC 1104 (Sharina et al., 1996 ) are probably the closest to the NGC 6789 in their nature.
The current stage of investigation of NGC 6789 has started with the search of new Local Volume galaxies by Karachentseva & Karachentsev ( 1998 ), who compiled the list of 260 dwarf galaxies on the basis of POSS-II and ESO-SERC survey. The NGC 6789 galaxy is outstanding by its high surface brightness in central regions, small radial velocity (according to ZCAT of Huchra ( 1995 ) it has Vo=-157 km/s), and spatial isolation. But, in spite of its optical high surface brightness, it was undetected in HI with 100m radiotelescope in Effelsberg (Huchtmeier et al., 1997 ).
Hence it was quite surprising to resolve this galaxy into stars with the 6 m telescope in April 1996 under rather poor seeing conditions. This allowed to make conclusion about its close proximity and excited interest to further observations. Later, on the basis of the July 1996 run with 6m telescope we have constructed a CM diagram of the NGC 6789, which shows the existence of two distinct groups of stellar population placed within different radii from galaxy optical center. In the CM diagram these groups stand out at two distinct loci, but the photometric limit in red frames was insufficient to locate the RGB.
Later optical spectral observations with 6m telescope long slit
spectrometer UAGS revealed an existence the HII gas in NGC 6789
(Karachentsev & Makarov, 1998 ).
The calculated radial H
-velocity proved to
be -141 km/s with the standard error ±8 km/s, that is close to ZCAT
value.
Deep photometric observations with Nordic 2.5m telescope in better seeing
allowed to reveal the RGB, while H frames have shown an existence of
large HII regions in the central region and 3 compact objects in outer.
The paper is organized as follows. Section 2 briefly presents the observations. In §3 we present the results of our work: in §3.1 an overview of the CM diagram is given and used to estimate the distance to NGC6789 (§3.3) and its metallicity (§3.2). In §3.4 the distribution of surface brightness is discussed. Finally, the results are summarized in §4.
- dark blue) of the central part of NGC 6789.
a) B,V,R,I,Ha composition of the original frame, b) B,V,I composition
the same part after subtracting the frame median smoothed with a window
of . The total field is 1.22'x1.38'.
North at left, East at up.