  We present the results of a detailed non-LTE analysis of the UV and optical
spectrum of the O6.5Iaf+ star HD153919 - the mass donor in the high-mass X-ray
binary 4U1700-37. Given the eclipsing nature of the system these results allow
us to determine the most likely masses of both components of the binary via
Monte Carlo simulations. These suggest a mass for HD153919 of 58+/-11M_sun -
implying the initial mass of the companion was rather high (>60 M_sun). The
most likely mass for the compact companion is found to be 2.44+/-0.27M_sun,
with only 3.5 per cent of the trials resulting in a mass less than 2.0M_sun and
none less than 1.65M_sun. Our observational data is inconsistent with the
canonical neutron star mass and the lowest black hole mass observed (>4.4M_sun;
Nova Vel). Significantly changing observational parameters can force the
compact object mass into either of these regimes but this results in the O-star
mass changing by factors of greater than 2, well beyond the limits determined
from its evolutionary state and surface gravity. The low mass of the compact
object implies that it is difficult to form high mass black holes through both
the Case A & B mass transfer channels and, if the compact object is a neutron
star, would significantly constrain the high density nuclear EoS
