  We present optical and ultraviolet observations of 3C273 covering the whole
life of the IUE satellite. We analyze the variability properties of the light
curves, and find that two variable components, written B and R respectively,
must contribute to the blue-bump emission in this object. The B component
produces most of the variability in the ultraviolet domain. A maximum time
scale of variability of about 2 yr identical at all wavelengths is found. If
discrete events produce this component, the event rate is ~3-30 yr^-1. Assuming
an isotropic emission, each event must liberate about 10^51 erg in the form of
optical-to-ultraviolet radiation. The spectral properties of the B component
suggest that reprocessing on a truncated disk, or partially- thick
bremsstrahlung may be the emission mechanism. We find evidence of a lag of a
few days between the light curves of the B component at optical and ultraviolet
wavelengths. Neither the variability properties, nor the spectral properties of
the R component can be accurately measured. This component varies on very long
time scales, and may completely dominate the historical light curve of 3C 273
in the optical domain. Combining knowledge from other wavelengths, we obtain
several indications that this component could reveal the ``blazar'' 3C 273. In
particular, its light curve is very similar to the hard X-ray light curve.
