Photometric observations of distant active comets

We present CCD VRC observations of 6 distant comets located at heliocentric distances of 3.4-7.2 AU. Time-series data were obtained on three nights in July, 2000 covering 16 hours. Each comet was observed after the perihelion , when a lower activity was expected. Contrary to expectation, we found wel l-defined circular comae and extended tails visible out to a few (3-5) arcm in. We detected a quasi-cyclic light variation of C/1999 J2, while C/1999 N 4 showed some hints of a more complex variation. C/2000 K1 was constant to +/-0.04 mag during the observing run. The standard V and RC data were used to estimate nuclear diameters, while the colour indices implied a slighly r eddish (V - R = 0.(m)68, C/2000 K1), a neutral (V - R = 0.(m)47, C/1999 N4) and a slightly bluish (V - R = 0.(m)25, C/1999 J2) coma. Simple fits of th e surface brightness distributions are also presented enabling order of mag nitude estimates of nuclear radii. Beside the time-series observations, fur ther single-shot observations of three faint comets are briefly described.