Optical and infrared observations of the Centaur 1997 CU26

Minor planet 1997 CU26 is a Centaur, and is probably undergoing dynamical e volution inwards from the Kuiper Belt. We present optical and infrared (VRI JHK) photometry which gives mean colours of V - R = 0.46 +/- 0.02, V - I = 1.02 +/- 0.02, V - J = 1.74 +/- 0.02, V - H = 2.15 +/- 0.02 and V - K = 2.2 5 +/- 0.02. The resulting relative reflectance spectrum lies between those of Chiron and Pholus (although closer to that of Chiron). A 1.6-2.6 mu m sp ectrum confirms the broad absorption feature at 2.05 mu m associated with w ater ice reported by Brown et al. 1997 CU26 displays no significant light c urve variation and (unlike Chiron) has no observable coma. We place an uppe r limit to the dust production rate of 1.5 kg s(-1). J-band data taken at p hase angles of 1.degrees 7 to 4.degrees 0 give a phase parameter of G(J) = 0.36 +/- 0.1, and are consistent with a phase parameter of G = 0.15 in the V band (a value often assigned to low-albedo objects when no other informat ion is available) if we assume a phase reddening of 0.017 mag deg(-1) in th e J band. We find V(1; alpha = 4.degrees 1) = 7.022 +/- 0.02, from which we deduce, by assuming G = 0.15 +/- 0.1, an absolute visual magnitude of H-V = 6.64 +/- 0.04.