Astronomy Astrophysics

Abstract. In 2000, comet 28P/Neujmin 1 was an ideal candidate for a phase function study: during its April opposition it reached a phase angle of = 0.8, and previous observations at similar heliocentric distances (r 4 AU) indicated that the comet was likely to be inactive. We observed this comet using ESO’s NTT and 2.2m telescopes at La Silla, on 6 epochs from April to August 2000, covering = 0.8{8. These data were combined with a large set of data from 1985 to 1997. In order to disentangle the rotation eects from the phase eects, we obtained complete rotation coverage at opposition, providing us with a lightcurve template. We have obtained an improved rotation period of 12:75 0:03 hrs, with a full amplitude of m = 0:45 0:05 mag, which yields an axis ratio lower limit of 1.51 0.07. For each subsequent epochs, we have obtained enough rotation coverage to re-synchronize the lightcurve fragments with the template, and determine the magnitude change caused by the phase function. The average colours of the nucleus, V −R = 0:454 0:050, and R − I = 0:413 0:050, are similar with those of D-type asteroids and other comet nuclei. The phase function obtained for the nucleus has a linear slope of 0:025 0:006 mag deg−1, less steep than that of the mean C-type asteroids