Null et al. (1993) have analyzed data from 7 HST orbits using the
wide-field mode of the WFPC-1 on August 4-6, 1991. The 
field of view of a single chip allows simultaneous observations of Pluto, Charon
and a background star, GSC 5006/98. They derived new values of mass and density
for the system, with a mass ratio of 0.0837
0.0147 and a mass of the system
of (1.401
0.019) 
10
g. Using radii derived by Tholen and
Buie (1990) from mutual eclipse data, they deduce densities of 2.13
0.04 g
cm
for Pluto and 1.30
0.23 g cm
for Charon.
A search for additional satellites of Pluto by Stern et al. (1994) has
yielded a null result. They have carried out simulations of a multibody system and derived constraints on the existence of such bodies. An effort to derive
individual rotational light curves for Pluto and Charon is underway. M. Buie,
D. Tholen and L. Wasserman (1994) obtained PC images over a period from May 21,
1992, to August 18, 1993 with the F555W (V-band) and the F439W (B-band) filters
in this study. A parallel study, spanning the same time interval, by L.
Trafton and A. Stern (1994) obtained FOS spectra with the G400H grating and a
0.5
aperture to exclude Charon. The goal of this investigation was to
obtain rotational light curve of Pluto as a function of wavelength in
ultraviolet light and to look for absorbing gas species.
Current NASA Headquarters plans for a mission to Pluto will motivate ongoing studies of this sort. With the refurbished telescope, separation of the two bodies is more easily achieved.