RED GIANTS WITH UNUSUAL DUST SHELLS .2. THE DUSTY R-STAR HD-100764

We present an 8-13 mu m spectrum of the unusual, dusty, early R star HD 100764. Contrary to earlier speculation this star does not have silicate dust around it, and is probably not directly linked evolutionarily to the controversial carbon stars with silicate dust features. The mid-IR spectrum, combined with existing optical and IR photometry, is used to model the dust around the star. We find that HD 100764 must have a massive (3 x 10(-5) M.) dust disc containing a distribution of grains ranging from small to very large, and that the observations are inconsistent with a spherical dust shell model. We argue that the most likely interpretation is that the star has acquired its dusty disc from a companion that has already evolved through the AGB and PN phases.