  In this Paper we report on phase resolved I-band optical spectroscopic and
photometric observations of CirX-1 obtained with the Very Large Telescope. The
spectra are dominated by Paschen absorption lines at nearly all orbital phases
except near phase zero (coinciding with the X-ray dip) when the absorption
lines are filled-in by broad Paschen emission lines. The radial velocity curve
of the absorption lines corresponds to an eccentric orbit (e=0.45) whose period
and time of periastron passage are consistent with the period and phase
predicted by the most recent X-ray dip ephemeris. We found that the I-band
magnitude decreases from 17.6 to ~16.8 near phase 0.9-1.0, this brightening
coincides in phase with the X-ray dip. Even though it is likely that the
absorption line spectrum is associated with the companion star of CirX-1, we
cannot exclude the possibility that the spectrum originates in the accretion
disc. However, if the spectrum belongs to the companion star, it must be a
supergiant of spectral type B5-A0. If we assume that the compact object does
not move through the companion star at periastron, the companion star mass is
constrained to ~<10 Msun for a 1.4 Msun neutron star, whereas the inclination
has to be ~> 13.7 degrees. Alternatively, the measured absorption lines and
their radial velocity curve can be associated with the accretion disc
surrounding a 1.4 Msun neutron star and its motion around the centre of mass.
An absorption line spectrum from an accretion disc is typically found when our
line-of-sight passes through the accretion disc rim implying a high
inclination. However, from radio observations it was found that the angle
between the line-of-sight and the jet axis is smaller than 5 degrees implying
that the jet ploughs through the accretion disc in this scenario.
