  We present optical spectra and images of the X-ray binary Circinus X-1. The
optical light curve of Cir X-1 is strongly variable, changing in brightness by
1.2 magnitudes in the space of four days. The shape of the light curve is
consistent with that seen in the 1980s, when the X-ray and radio counterparts
of the source were at least ten times as bright as they are currently. We
detect strong, variable H-alpha emission lines, consisting of multiple
components which vary with orbital phase. We estimate the extinction to the
source from the strength of the diffuse interstellar bands and the Balmer
decrement; the two methods give A_V = 7.6 +/- 0.6 mag and A_V > 9.1 mag
respectively. The optical light curve can be modelled as arising from
irradiation of the companion star by the central X-ray source, where a
low-temperature star fills its Roche lobe in an orbit of moderate eccentricity
(e ~ 0.4). We suggest that the companion star is over-luminous and under-dense,
due to the impact of the supernova which occurred less than 5000 yr ago.
