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We present the first case of strong gravitational lensing by a QSO : SDSSJ0013+1523, at z = 0.120. The discovery is the result of a systematic searchfor emission lines redshifted behind QSOs, among 22298 spectra of the SDSS datarelease 7. Apart from the z = 0.120 spectral features of the foreground QSO,the spectrum of SDSS J0013+1523 also displays the OII and Hbeta emission linesand the OIII doublet, all at the same redshift, z = 0.640. Using sharp Keckadaptive optics K-band images obtained using laser guide stars, we unveil twoobjects within a radius of 2 arcsec from the QSO. Deep Keck opticalspectroscopy clearly confirms one of these objects at z = 0.640 and showstraces of the OIII, emission line of the second object, also at z = 0.640. Lensmodeling suggests that they represent two images of the same z = 0.640emission-line galaxy. Our Keck spectra also allow us to measure the redshift ofan intervening galaxy at z = 0.394, located 3.2 arcsec away from the line ofsight to the QSO. If the z = 0.120 QSO host galaxy is modeled as a singularisothermal sphere, its mass within the Einstein radius is M_E(r < 1 kpc) =2.16e10 M_Sun and its velocity dispersion is sigma_SIS = 169 km/s. This isabout 1 sigma away from the velocity dispersion estimated from the width of theQSO Hbeta emission line, sigma_*(M_BH) = 124 +/- 47 km/s. Deep optical HSTimaging will be necessary to constrain the total radial mass profile of the QSOhost galaxy using the detailed shape of the lensed source. This first case of aQSO acting as a strong lens on a more distant object opens new directions inthe study of QSO host galaxies.

First case of strong gravitational lensing by a QSO : SDSS J0013+1523 at z = 0.120