  We present the results of simultaneous X-ray and radio observations of the
peculiar Z-type neutron star X-ray binary Cir X-1, observed with the Rossi
X-ray timing explorer satellite and the Australia Telescope Compact Array in
2000 October and 2002 December. We identify typical Z source behaviour in the
power density spectra as well as characteristic Z patterns drawn in an X-ray
hardness-intensity diagram. Power spectra typical of bright atoll sources have
also been identified at orbital phases after the periastron passage, while
orbital phases before the periastron passage are characterized by power spectra
that are typical neither of Z nor of atoll sources. We investigate the coupling
between the X-ray and the radio properties, focusing on three orbital phases
when an enhancement of the radio flux density has been detected, to test the
link between the inflow (X-ray) and the outflow (radio jet) to/from the compact
object. In two out of three cases we associate the presence of the radio jet to
a spectral transition in the X-rays, although the transition does not precede
the radio flare, as detected in other Z sources. An analogous behaviour has
recently been found in the black hole candidate GX 339-4. In the third case,
the radio light curve shows a similar shape to the X-ray light curve. We
discuss our results in the context of jet models, considering also black hole
candidates.
