Distance, structure and bright stellar content of the dwarf irregular galaxy UGC 685

B and R CCD images and J NICMOS3 frames taken with the Calar Alto 3.5 m tel escope of the dwarf irregular galaxy UGC 685 are presented. The brightest p art of the stellar population is resolved in B and R, very few also in J. T he stellar color-magnitude diagram is discussed. An estimate of the distanc e to UGC 685 of 5.5 Mpc is derived based on the brightest blue supergiant s tars. Most of the resolved bright stars show colors in the range -0.1 less than or equal to B - R less than or equal to +0.7. The continuum light of the unresolved stars is distributed rather regular i n B, R, and J, showing only minor irregularities. This surface brightness d istribution follows an exponential law in all three colors with a scale len gth r(c) = 0.33 kpc. The central surface brightnesses are 21.57 +/- 0.09 ma g/square ", 20.65 +/- 0.06 mag/square ", and 20.11 +/- 0.11 mag/square ", i n B, R, and J, respectively. The surface brightness can be traced out to 5 r(c) in B and R. Thus, UGC 685 belongs to the class of dwarfs where the HI distribution is much more extended (here 2.6 times) than the optical (stell ar) light distribution, but contrary to many objects of this type, it does not belong to the class of low-surface brightness objects. No color gradien t was detected in UGC 685 except that the very center is slightly bluer. Th e overall colors are B - R = +0.971 B - J = +1.55 and the magnitude is B = 14.55 (M-B(0) = -14.5). The classification as an irregular dwarf from survey plates results from th e few HII regions of UGC 685 which are all concentrated to the South-East o f the center of the galaxy. On an H alpha Calar Alto 2.2 m telescope CCD im age, I identified only 5 HII regions, one of them being rather bright. The total Hoc flux corresponds to a mildly on-going star formation with a rate of 0.003 M. yr(-1), a low rate even in comparison to other dwarfs. The avai lable (and limited) data do not indicate any major deviation from this rath er low star formation rate within the last 10(9) yr.