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tokens: for further analyses. Fig. 2 shows the final image after co-adding. It clearly shows a central galaxy surrounded by an extend blueish arc in the south-west (labelled as C and D) and two other blobs in the north (A) and south-east (B), respectively. Both the central galaxy and the extended arc are observed spectroscopically by the SDSS, and their spectra are shown in the right-hand panel of Fig.1. With the aid of the better imaging resolution provided by the CFHT/Megacam, the blueish arc is resolved into two blobs (labelled as C and D). Furthermore, another faint blob in the south-east (B) and a red, almost full Einstein ring, are clearly detected for the first time. This is certainly a strong lensing system of quadruple images, and the background galaxy contains a quasar which is very special because we can see not only the quasar but also its host galaxy. Figure 1. Left-hand panel: The SDSS image of the lens system. The fibre subtend an angle of 2 arcsec (white circle). Right-hand panels: The SDSS-measured spectra of the lens galaxy (top) and the lensed quasar (bottom). Figure 2. The co-added data of g band, r band and i band taken by CFHT. 3 LENS MODELLING STRATEGY The system is modelled by the lens modelling tool lfit_gui developed by Shu etal. ( 2016c ). Here we briefly describe the settings used in this work. 3.1 Foreground-light and source models We model the surface brightness distribution of the lens galaxy with elliptical Srsic profile defined as in which I e is the surface brightness at the effective radius r e that encloses half of the total light. The constant b ( n ) is defined in Ciotti Bertin ( 2012 ) and MacArthur etal. ( 2003 ) 4 . As the CFHT data clearly resolve the Einstein ring formed from the quasars host galaxy, we model the source surface brightness distribution with one elliptical Srsic model. We choose not to model the quasar itself because the Einstein ring already provides robust constraints on the lens model. 3.2 Lens model The projected surface mass distribution of lens galaxy is modelled by singular isothermal ellipsoid profile (hereafter SIE) (for details, see Kormann, Schneider Bartelmann 1994 ). Let denote the two-dimensional position of source on the source plane and the impact vector of light ray in the lens plane, both measured with respect to an optical axis. The gravitational lensing equation shows where D d , D s and D ds denote the angular diameter distance to the lens, the source and from the lens to the source, respectively. is the deflection angle, related to the surface mass density by Introducing angular coordinates by we can transform equation ( 2 ) to the interpretation of the lens in equation ( 5 ) is that a source with true position can be seen by an observer to be located at angular positions satisfying equation ( 5 ). So, the dimensionless lens equation is obtained as where , and length scale 0 are defined as and SIE is the lensing velocity dispersion. We can express the scaled deflection