1 Introduction  

3C273 is the brightest quasar on the sky, with a mean V band magnitude of 12.9. It can be easily observed from both hemispheres, thanks to its position very close to the celestial equator: $\alpha$=12$^{\rm h}$29$^{\rm m}$06.70$^{\rm s}$,$\delta$=+02$\hbox{$^\circ$}$03$\hbox{$^\prime$}$08.6$\hbox{$^{\prime\prime}$}$ (J2000.0) and at high galactic latitude: l=289.95, b=+64.36. The redshift of 3C 273 is z=0.158, which corresponds to an effective distance of 440h^-1Mpc, with a Hubble constant defined as $H_0=100\,h\,\mbox{km\,s$^{-1}$\,Mpc$^{-1}$}$and a deceleration parameter of q₀=0.05. The mean bolometric flux received from 3C 273 between 10⁷ and 10²⁵Hz (see Fig. 6) is 1.910^-9ergcm^-2s^-1. If the whole spectrum of 3C 273 is emitted isotropically, this flux would correspond to a bolometric luminosity of 6.010$^{46}\,h^{-2}$ergs^-1. The host galaxy of 3C 273 is an elliptical (E4) galaxy, which has an outer radius of about 15$\hbox{$^{\prime\prime}$}$ and a V band magnitude of 16.4 (Bahcall et al. 1997).

3C 273 has a jet with apparent superluminal motion that ends with a hot spot called 3C273A. It is among the few active galactic nuclei (AGN) detected at energies above 100MeV by the Energetic Gamma-Ray Experiment Telescope (EGRET) on board the Compton Gamma-Ray Observatory (CGRO) and is thus often classified as a blazar despite its prominent blue-bump and its strong emission-lines. This coexistence of blazar- and Seyfert-like properties makes 3C 273 be a very interesting but complex object.

The detection of strong ultraviolet variability in 1982 (Courvoisier & Ulrich 1985) was at the origin of the multi-wavelength monitoring campaign that started in 1983. This observation effort, which is still going on, led to several publications on the multi-wavelength properties of 3C 273 (Courvoisier et al. 1987, 1990; Lichti et al. 1995; von Montigny et al. 1997). More detailed variability studies in specific spectral domains were also possible with these observations (e.g. Turner et al. 1990; Cappi et al. 1998 (X-rays); Ulrich et al. 1988 (ultraviolet); Courvoisier et al. 1988 (optical and infrared); Robson et al. 1993 (infrared to radio); Stevens et al. 1998 (millimetre to radio)).

The end of the IUE operations and the writing of a review paper on 3C 273 (Courvoisier 1998) is a good opportunity to make all these data available. The aim of this contribution is to present and to maintain a high quality publicly available database of most observations of 3C 273. Publishing such a wide database is not a goal in itself, but has the purpose to stimulate variability analyses by a large community of astronomers. The huge effort of observing 3C 273 during more than 30 years should lead to a better understanding of this object and hence of AGN in general. The detailed studies of the blue-bump variability (Paltani et al. 1998) and of the millimetre-to-radio flaring behaviour (Türler et al. in preparation) are examples of what can be done with these data and we hope that this analysis effort will continue elsewhere, thanks to this database.