Detailed observations of NGC 4151 with the IUE -II. Variability of the continuum from 1978 february to 1980 may, including X-ray and optical observations.

NGC 4151 has been extensively monitored with IUE from 1978 February to 1980 May. The rather erratic behaviour of the ultraviolet light curve seems to be mainly due to variations which occur at rates which, if extrapolated, would produce factor two changes in time-scales between five and 30 days, implying a radius of the order of 0.01 pc for the source. The shape of the continuum can be described by a power law longward of λ2200 and by an excess above the extrapolation of that law at shorter wavelengths, suggesting the presence of two components. The long wavelength spectrum becomes harder when the flux increases. The relationship of the short wave excess to the long wavelength component depends on the reddening assumed in the fitting and extrapolation of the power law. According to the assumptions made, the excess is either constant or anticorrelated with the long wavelength flux but a small number of "anomalous" states exist when the short wavelength excess is absent or weaker than otherwise expected.