  Hubble Space Telescope (HST) limits on the amount of halo dark matter (DM) in
the form of very low-mass (VLM) stars close to the hydrogen-burning limit have
been interpreted as excluding such stars as viable candidates. However, these
limits assume that the stars are smoothly distributed and have at least the
metallicity of Population II stars, whilst some baryonic DM formation theories
predict that they may instead be clumped into globular-cluster configurations
and have close to zero metallicity. I re-analyse the HST data employing the
zero-metallicity VLM star models of Saumon et al. (1994), which predict V-I
colours below the cuts of previous analyses for stars below 0.2 Mo. From the
models I derive new limits on the allowed halo fraction comprising VLM stars
for both the unclustered and clustered cases. In the unclustered regime I find
a 95% confidence upper limit on the allowed halo fraction of 1.4% inferred from
20 HST fields, comparable to limits derived by previous studies for non-zero
metallicity populations. In the cluster scenario I show that clusters of mass M
and radius R can satisfy both HST and the recent MACHO gravitational
microlensing results, which indicate a lens halo fraction of 40% for a standard
halo model, provided R < 1.2 (M/10^4 Mo)^0.74 pc. However, existing dynamical
limits restrict the allowed range to a tiny region characterised by M \sim
4x10^4 Mo and R \sim 3 pc. Furthermore, consistency between MACHO and HST
demands a present-day clustering efficiency of 92% or better. Intriguingly
however, the cluster mass implied by these limits is theoretically well
motivated and the VLM star scenario may also help to provide an explanation for
the faint red `halo' light recently reported around another galaxy.
