Introduction

Studies of asteroid rotation have shown that asteroids larger than a few hundred meters in diameter rotate with periods greater than 2.2 h;the barrier is interpreted as evidence that the asteroids are mostly loosely bound, gravity dominated aggregates, called ``rubble piles'' (Harris 1996, Pravec and Harris 1999). Recent optical and radar observations of small, near-Earth asteroids 1998 KY₂₆ and 1995 HM have shown that they rotate with periods of 10.7 min and (most likely) 97.2 min, respectively. These data indicate that 1998 KY₂₆ and 1995 HM are monolithic bodies (Ostro et al. 1999, Steel et al. 1997). With absolute magnitudes of H=25.5 and 22.5, respectively, the two fast rotators are the smallest asteroids among those with estimated periods. The present statistics suggest that the limiting size below which monoliths start to dominate the asteroid population is close to the absolute magnitude H=22, corresponding to a diameter of about 0.2 km (Pravec and Harris 1999).

We present observations and analysis of three more asteroids with H>22. All have short rotation periods in the range 2-20 minutes, and must be monolithic in nature.