VARIABLE PHASE-FACTORS DURING THE ROTATION OF ASTEROID-51 NEMAUSA

The phase factor varied between 0.032 and 0.045 mag/deg on the differe nt sides shown during the rotation of the asteroid 51 Nemausa in the 1 990 and 1991 oppositions. The effect is large enough to change the sha pe of the lightcurves and complicates the pole determination problem. Color variations with amplitudes 0.015 mag in u-b and 0.004 mag in v-y confirm the variations found in 1989. The effects are single periodic with one extremum and are caused by differences in composition and te xture over the surface. Differences between the opposite pairs 1990-19 85 and 1983-1989 show identical deviations from the theoretical sine-c urve. A new procedure is presented for differential photometry. A fict itious comparison star with the same airmass as the planet is used as reference during the entire observation period.