The time delay $\Delta t = 130\pm3$ days (1 $\sigma$ ) has been measured for the first time, in the lensed quasar SBS 1520+530 on the basis of R-band images obtained with the NOT. Keck spectroscopy of the lensing galaxy strongly suggests that the absorption system at z=0.717 is associated with the lensing galaxy.

Applying the detailed mass model presented by Faure et al. (2002) we derive a mean value $H_0= 51 \pm 9~\rm km~ s^{-1}~Mpc^{-1}$ . When only the main lensing galaxy is used in the mass model H₀ increases to $63 \pm 9~\rm km~s^{-1}~Mpc^{-1}$ . The fit of the two different models depends on the flux ratio between the two images, the model with only one galaxy only gives a good fit when the high flux ratio measured from the emission lines is applied. Determining precisely the flux ratio is therefore crucial to further improve the modeling of the system and hence the precision on H₀. Finally, a possible systematic error adds to the uncertainty in H₀ if the central mass concentration is not isothermal.

Table 1: Lens models: the first column describes the lens parameters, where $\gamma$ and $\theta _{\gamma }$ are the intensity and orientation of the shear at the position of the quasar images. Columns 2 and 3 gives the models results for the two possible flux ratio measured in SBS 1520+530. The letter $\sigma$ refers to lens velocity dispersions and $r_{\rm cut}$ is the cut radius of the different mass distributions.
$M_{\rm B} - M_{\rm A}$ 0.83 mag 1.4 mag

L no good fit $\sigma_{\rm L}$ = 228 km s^-1 $r_{\rm cut}$ (L) = 13 kpc

$\gamma$ 0.39

$\theta _{\gamma }$ +75.3 $\deg$

H₀ (1 $\sigma$ ) 63 $\pm$ 9 km s^-1Mpc^-1

$\sigma_{\rm L}$ = 189 km s^-1 $r_{\rm cut}$ (L) = 51 kpc $\sigma_{\rm L}$ = 200 km s^-1 $r_{\rm cut}$ (L) = 100 kpc

L+M+C $\sigma_{\rm M}$ = 135 km s^-1 $r_{\rm cut}$ (M) = 20 kpc $\sigma_{\rm M}$ = 118 km s^-1 $r_{\rm cut}$ (M) = 40 kpc

$\sigma_{\rm C}$ = 718 km s^-1 $r_{\rm cut}$ (C) = 600 kpc $\sigma_{\rm C}$ = 400 km s^-1 $r_{\rm cut}$ (C) = 600 kpc

$\gamma$ 0.32 0.34

$\theta _{\gamma }$ +74.5 $\deg$ +74.3 $\deg$

H₀ (1 $\sigma)$ 52 $\pm$ 8 km s^-1Mpc^-1 50 $\pm$ 8 km s^-1Mpc^-1

**Table 1:** Lens models: the first column describes the lens parameters, where $\gamma$ and $\theta _{\gamma }$ are the intensity and orientation of the shear at the position of the quasar images. Columns 2 and 3 gives the models results for the two possible flux ratio measured in SBS 1520+530. The letter $\sigma$ refers to lens velocity dispersions and $r_{\rm cut}$ is the cut radius of the different mass distributions.
$M_{\rm B} - M_{\rm A}$	0.83 mag	1.4 mag
L	no good fit	$\sigma_{\rm L}$ = 228 km s^-1 $r_{\rm cut}$ (L) = 13 kpc

$\gamma$		0.39
$\theta _{\gamma }$		+75.3 $\deg$
H₀ (1 $\sigma$ )		63 $\pm$ 9 km s^-1Mpc^-1
	$\sigma_{\rm L}$ = 189 km s^-1 $r_{\rm cut}$ (L) = 51 kpc	$\sigma_{\rm L}$ = 200 km s^-1 $r_{\rm cut}$ (L) = 100 kpc
L+M+C	$\sigma_{\rm M}$ = 135 km s^-1 $r_{\rm cut}$ (M) = 20 kpc	$\sigma_{\rm M}$ = 118 km s^-1 $r_{\rm cut}$ (M) = 40 kpc
	$\sigma_{\rm C}$ = 718 km s^-1 $r_{\rm cut}$ (C) = 600 kpc	$\sigma_{\rm C}$ = 400 km s^-1 $r_{\rm cut}$ (C) = 600 kpc
$\gamma$	0.32	0.34
$\theta _{\gamma }$	+74.5 $\deg$	+74.3 $\deg$
H₀ (1 $\sigma)$	52 $\pm$ 8 km s^-1Mpc^-1	50 $\pm$ 8 km s^-1Mpc^-1

External variations, probably due to microlensing effects, are observed on the time delay shifted light curves (Fig. 4). Part of this effect can be modeled as a linear term, or corrected for with the iterative algorithm for measuring time delays. However, significant external variations of time scales of $\sim$ 50 days do remain once these corrections are made.

Microlensing is important in SBS 1520+530 as suggested both by the high frequency variations in the light curves and by the spectra of the quasar images. With light curves in only one band we can not efficiently disentangle between microlensing and intrinsic variations of the quasar. Such a work shall be possible using the colour information provided by a monitoring of the object in spectroscopy. The issue in conducting such a spectrophotometric monitoring for SBS 1520+530 is double: (1) to measure the true flux ratio between the quasar images and discriminate between lens models and (2) to use microlensing to infer constraints on the distribution of micro-lenses in galaxy (L) and/or to reconstruct the energy profile of the central AGN in the source.

7 Discussion