We have presented a B, V, R, I photometry of 337 resolved stars in the blue compact dwarf galaxy NGC 6789. The galaxy shows CM diagram where the main features are two stellar populations distinct in structure and colour: underlying older low-surface-brightness stellar population formed previously to the present burst, and an inner high-surface-brightness young population within 150 pc from the center of the galaxy superimposed on the older stellar component.
The tip of the first ascent RGB is used to estimate a metallicity and
distance of the NGC 6789. A splitting of the upper part of the RGB
prevents a unique determination and forces the assumption of two
possible values for the metallicity are [Fe/H] 
and
[Fe/H] 
.
But both values give very close distance estimations, about 2.1 Mpc.
This is consistent with its position in the VSun versus
cos
diagram, at what is considered to be the outskirts
of the Local Group.
NGC 6789 has a light distribution which is well described by combination
of the gaussian and exponential laws.
Its central surface brightness is 
,
and its isophotal equivalent diameter
is 
or D26,equiv=1.2 kpc,
making it an intrinsically small high surface brightness galaxy.
NGC 6789's integrated absolute magnitude within
is
.
It is also quite blue
((V-I)0=0.6) in its central region, but its halo much redder, so
the total colour index of NGC 6789 inside isophote
is (V-I)0=0.9.
The nature of BCDs remains ambiguous and confusing (Papaderos et al. 1996); thus, theories to explain their nature are very controversial. There are different competing hypothesis of evolutionary connections between BCDs, dIs and dEs.
The detection of an extended faint stellar underlying component (Loose & Thuan 1985, 1986; Kunth et al. 1988), in the majority of BCDs supports the idea that they are not truly primordial galaxies, but older LSB dwarf galaxies undergoing transient periods of star formation. If dIs were to end up as dEs after going through repetitive BCD phases, their underlying LSB stellar component must modify its structural properties (Papaderos et al. 1996). This may occur in response to changes in the global gravitional potential of the dwarf galaxy, caused by such events as mass infall from the outer gas halo and/or mass loss in starburst-driven galactic winds. In the case of NGC 6789 the continous gas infall from the halo could lead to the present picture: the presence of rather large amounts of the ionized hydrogen in the central region of the galaxy inspite of the lack of detected HI gas.
Although our results cannot clearly designate the direction of evolution, they suggest some evolutionary relation between dwarf galaxies (dIrs,BCDs, and dEs).
It will be interesting to study nearby resolved isolated BCDs, as the NGC 6789. It is accessible to be studied in detail, including the ages, metallicities and distributions of its stellar populations, and their structural parameters. More deep photometry is need to make more precise conclusion about its star formation history.