Ostro, S. J., J. F. Chandler, A. A. Hine, I. I. Shapiro, K. D. Rosema, and D. K. Yeomans. (1990). Science 248, 1523-1528.
Radar observations of the near-Earth asteroid 1989 PB, made shortly after its optical discovery, yield a sequence of delay-Doppler images that reveal it to consist of two distinct lobes that appear to be in contact. It seems likely that the two lobes once were separate and that they collided to produce the current "contact-binary" configuration.