CCD astrometry of southern very low-mass stars

CCD astrometry of a sample of southern very low-mass (VLM) stars, obtained on the 2.2-m telescope at the European Southern Observatory, La Silla, is presented. Trigonometric parallaxes have been determined for 13 stars, and proper motions for a further two stars. Photometry in the I and K passbands is also presented for the astrometric programme stars, and is used to construct an MK:I — K colour-magnitude diagram. Astrometric observations have been made of the canonical very late M dwarf VB10 through a narrow-band (Δλ = 90 Å) filter. These data confirm that the effects of differential colour refraction can be substantially reduced by the use of such filters, without compromising astrometric precision.