  We observed the Fermi-discovered gamma-ray binary 1FGL J1018.6-5856 at 20
epochs over 50 days using the CHIRON spectrograph, obtaining spectra at
R~25,000 covering 4090-8908A. The average spectrum confirms an O6 V((f))
spectral type and extinction E(B-V) = 1.35+/-0.04. Variable absorption line
equivalent widths suggest substantial contamination by wind line features. The
limited S/N ratio hindered accurate continuum definition and prevented
measurement of a high quality radial velocity curve. Nevertheless, the best
data indicate a radial velocity amplitude <40 km/s for the He II lines and
substantially lower for H I. We argue that this indicates a most likely compact
object mass <2.2Msun. While black hole solutions are not excluded, a neutron
star source of the gamma-ray emission seems preferred.
