Orbital modulation and longer term variability in the radio emission from Cygnus X-1

20 months of observations of the radio emission at 15 GHz from Cygnus X-l, starting in 1996 October, show variations at the binary period of 5.6 d, bu t with a phase offset from those at Xray wavelengths. There are also longer term variations on a time-scale of 150 d, which are only loosely related t o the soft X-ray flux. The source was in the hard/low X-ray state throughou t this period. The mean 15-GHz flux density is 13 mJy, the radio spectrum i s flat and the semiamplitude of the orbital modulation is about 2 mJy. We d iscuss the possible origins of the modulation and the relationship to the s oft X-ray emission.