Simultaneous radio and optical observations of Circinus X-1.

Summary. Two sets of observations of the Circinus X-l radio source are presented: at 14 GHz during the outburst of 1977 March 6-7 and at 1.4, 2.3, 5.0 and 8A GHz during the outburst of 1977 May 12-15. At 5 GHz the outburst had less than 1 per cent linear polarization. A high-resolution map shows an unresolved bright radio source superimposed on a weak extended region. Enhanced optical emission was observed during the May 12-13 radio outburst. The evolution of the radio spectrum and the relationship of Cir X-1 to the SNR G321.9 - 0.3 to the south are discussed.