Plot of the Ångström exponent for spherical grains of a given radius for various materials, and for an index of refraction of n = 1.6 and for various imaginary components of the index of refraction, k. The horizontal dashed lines give the 1 and 2σ limits on the ratio of the transit depths, D, between our V- and R-band observations. Our limits on the Ångström exponent allow us to state that the radius of single sized grains in the dusty tails streaming behind these planetesimals must be ∼0.15 μm or larger, or ∼0.06 μm or smaller, with 2σ confidence.