Cometary activity at 25.7 AU: Hale--Bopp 11 years after perihelion

Eleven years after its perihelion, comet C/1995 O1 (Hale-Bopp) is still active. Between October 20–22, 2007, we detected a diffuse coma of 180x10^3$km in diameter with a slight elongation toward N/S direction. The integrated brightness was 20.04 mag in R_C, implying Af\rho=300 m and albedo x dust surface $a_{R}C$=4300 km^2. The coma was relatively red at V-R=0.66 mag, which is consistent with that of the dust in other comets. The observed properties and the overall fading in brightness between 10 AU and 26 AU follow the predicted behaviour of CO-driven activity (Capria et al. 2002). This is the most distant cometary activity ever observed.