Variations of Circinus X-1 in the infrared.

Summary. JHKLM observations of the Mayo et al. optical candidate for Cit X-l show that it varies with the same period as the X-ray source, thus securing the identifIcation. The shape of the light curve is unique, a sudden rise near the time of the X-ray drop being followed by a gradual decay. The colour of the source remains constant.