LONG-TERM INFRARED BEHAVIOR OF CIR X-1

Cir X-1 has been observed at SAAO in the JHKL region on many occasions covering a time span of 15 years. A maximum in the infrared flux is always observed at binary phase 0 according to the radio-derived ephemeris, even though the radio flares are now frequently undetectable. The infrared luminosity at maximum declined between 1977 and 1980 but has remained fairly stable since that time. Between maxima, the source appears to be in a quasi-constant state. The duration of the rise to maximum is about 1.5 d and that of the decline about 3 d. The spectrum of the variable part stays constant in shape in spite of the changes in overall intensity. The unusually red colours of the source, even after de-reddening, make the interpretation of the data difficult and to some extent ambiguous.