We present the results of a 2.5-year multiwavelength monitoring programme of
Cygnus X-1, making use of hard and soft X-ray data, optical spectroscopy,
UBVJHK photometry and radio data. In particular we confirm that the 5.6-day
orbital period is apparent in all wavebands and note the existence of a
wavelength-dependence to the modulation, in the sense that higher energies
reach minimum first. We also find a strong modulation at a period of 142 +/- 7
days, which we suggest is due to precession and/or radiative warping of the
accretion disc. Strong modulation of the hard and soft X-ray flux at this long
period may not be compatible with simple models of an optically thin accretion
flow and corona in the low state. We present the basic components required for
more detailed future modelling of the system - including a partially optically
thick jet, quasi-continuous in the low state, the base of which acts as the
Comptonising corona. In addition, we find that there are a number of flares
which appear to be correlated in at least two wavebands and generally in more.
We choose two of these flares to study in further detail and find that the hard
and soft X-rays are well-correlated in the first and that the soft X-rays and
radio are correlated in the second. In general, the optical and infrared show
similar behaviour to each other but are not correlated with the X-rays or
radio.Comment: Accepted for publication in MNRAS, 2 figures in colou