1 Introduction

Figure 1: NGC 6789 in the I band. a) The original frame, b) the same part after subtracting the median, smoothed with a window of $10\times (FWHM)$. The size of the CCD frame is 3.7 by 3.7 arcmin. North left, East top. The isophotes correspond to $22.5~{\rm mag/\ifmmode\hbox{\rlap{$\sqcap$ }$\sqcup$ }\else{\unskip\nobreak\hfi... ... } \parfillskip=0pt\finalhyphendemerits=0\endgraf}\fi\hbox{$^{\prime\prime}$ }}$ and $24.2~{\rm mag/\ifmmode\hbox{\rlap{$\sqcap$ }$\sqcup$ }\else{\unskip\nobreak\hfi... ... } \parfillskip=0pt\finalhyphendemerits=0\endgraf}\fi\hbox{$^{\prime\prime}$ }}$. They are used to define the central region (C), the perefery (E) and the foreground stars (F)

In spite of its New General Catalog membership this galaxy is poor studied.

NGC 6789 is in the list of 260 new Local Volume galaxies by Karachentseva & Karachentsev ([1998]), who used the POSS-II and ESO-SERC survey. The NGC 6789 galaxy is outstanding by its high surface brightness in central region, small radial velocity (V₀=-157 km s^-1 Huchra [1995]), and spatial isolation. But, in spite of its optical high surface brightness, it was undetected in H I with the Effelsberg 100 m radiotelescope (Huchtmeier et al. [1997]).

We resolved this galaxy into stars with the 6 m telescope in April 1996 under rather poor seeing conditions. In a July 1996 run with the 6 m 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. The photometric limit in the red frames did not allow to locate the RGB.

Later optical spectral observations with the 6 m-telescope long slit spectrometer UAGS revealed H II gas in NGC 6789 (Karachentsev & Makarov [1998]). The H$\alpha$ radial velocity is $-141\pm8$ km s^-1, close to value of Huchra ([1995]).

Deep photometric observations with Nordic 2.5 m telescope with better seeing revealed the RGB, while H$\alpha$ frames have shown large H II regions in the central region and 3 compact objects in the outer region.

Section 2 briefly presents the observations. In Sect. 3 we present the results of our work: in Sect. 3.1 an overview of the CM diagram is given and used to estimate the distance to NGC 6789 (Sect. 3.3) and its metallicity (Sect. 3.2). In Sect. 3.4 the distribution of surface brightness is discussed. Finally, the results are summarized in Sect. 4.

Figure 2: Images of the central part of NGC 6789. a) An continuum-subtracted H$\alpha$ image, b) the V band image after subtracting the median, smoothed with a window of $10\times (FWHM)$. The total field is $1.22\hbox {$^\prime $ }\times 1.38\hbox {$^\prime $ }$. North left, East top