of observational data on this population, including optical and near-infrared photometry; optical and near-infrared spectroscopy; and measurement of their clustering properties (Steidel, Pettini & Hamilton, 1995; Shapley et al., 2001; Shapley et al., 2003; Pettini et al., 2001; Adelberger et al., 1998; Giavalisco & Dickinson, 2001). Nonetheless, the most fundamental quantity – the dark–matter halo virial mass, or equivalently the circular velocity at large radius – is poorly determined for Lyman–break galaxies (LBGs). This is because it requires observations in the very faint outer regions of galaxies of rest–frame optical emission lines, which are redshifted to the near–infrared where the night sky is bright. As a consequence only a handful of rotation curves, with low signal–to–noise and compromised by seeing, have been measured for LBGs. This paper is concerned with extracting the maximum useful information on the virial masses of the haloes of LBGs from the existing kinematic data. Throughout the paper the term LBG refers to galaxies at $z=3$ identified by their spectral discontinuity at the Lyman–limit, that are brighter than ${\mathcal R}=25.5$ (Adelberger et al., 1998).

Lacking high quality rotation curves, most of our knowledge of the virial masses of LBGs comes indirectly, from an interpretation of their 