RX J0911.4+0551: A new multiple QSO selected from the ROSAT All-Sky Survey.

We report from follow-up observations of high redshift AGN candidates from the ROSAT All-Sky Survey (RASS) that RX J0911.4+0551 is a gravitationally lensed QSO with z=2.800. With an X-ray luminosity of L_X_=4.10^46^ergs/s it belongs to the X-ray brightest radio quiet QSOs. However, we must point out that the existing observations do not exclude the possibility that other objects inside the error circle are responsible for the X-ray flux, in particular a lensing cluster. Careful image analysis discerned three optical images. The separation between the two brighter components of 0.80" is below the seeing value during the observations. The fainter component has a distance of 3.1". The spectra of the brighter joint A, B component and the C component show no measurable differences in continuum and absorption lines. Three strong CIV absorption line systems with W_lambda_(1549)>1A are visible in the spectra. All three have z>2.3 which makes it improbable that they are caused by the lensing systems. The only spectral difference are larger equivalent widths of the broad emission lines in the brighter QSO components.