AND

matches a new value of 0.124 6 0.008 by Null and Owen (1996, Astron. J. 111, 1368–1381). The mass ratio and resultingThe Charon/Pluto mass ratio is a fundamental but poorly individual masses and densities of Pluto and Charon are consis-known parameter of the two-body system. Previous values for tent with a collisional origin for the Pluto–Charon sys-the mass ratio have ranged from 0.0837 6 0.0147 (Null et al., tem. 1997 Academic Press1993, Astron. J. 105, 2319–2335) to 0.1566 6 0.0035 (Young et al., 1994, Icarus 108, 186–199). We report here a new determi-nation of the Charon/Pluto mass ratio, using five sets of INTRODUCTIONgroundbased images taken at four sites in support of Pluto occultation predictions. Unlike the Null et al. and Young et al. The masses of Pluto and Charon provide key informa-determinations, where the centers of light for Pluto and Charon tion regarding the composition and origin of the two bod-could be determined separately, this technique examines the motion of the center of light of the blended Pluto–Charon ies. Given their radii, the densities of the two bodies ca