1 Introduction

Prompted by the successful optical measurements of time delays in PG 1115+080 (Schechter et al. 1997), a photometric monitoring campaign of gravitationally lensed quasars has been carried out at the Nordic Optical Telescope (NOT) with the aim of measuring time delays for many gravitationally lensed quasars. This observable is of crucial importance to study the mass distribution in lensing galaxies and to determine H₀ using the method proposed by Refsdal (1964).

Two time delays have been measured so far at the NOT: one for the doubly imaged quasar B 1600+434, lensed by an edge-on spiral (Burud et al. 2000) and another for the two (summed) components RX J0911+0550 (Hjorth et al. 2002). In this paper we will present our third time delay measurement: that of SBS 1520+530, a doubly imaged BAL quasar at z=1.86. SBS 1520+530 was discovered by Chavushyan et al. (1997) as a double quasar with angular separation of 1.56 $^{\prime\prime}$. The lensing galaxy was detected by Crampton et al. (1998) on H-band images obtained by using the adaptive optics system of the Canada-France-Hawaii Telescope. The lensing galaxy is also well resolved on optical images obtained at the NOT and deconvolved using the MCS deconvolution algorithm (Magain et al. 1998), and on public Hubble Space Telescope near-IR data (Faure et al. 2002). The redshift of the galaxy however, remains unknown. With the aim of measuring this redshift, we have obtained a spectrum of SBS 1520+530 with ESI at the Keck observatory.

The observations and data reduction of the images are presented in Sects. 2 and 3. The time delay measurement is described in Sect. 4 while the analysis of the spectroscopic data is explained in Sect. 5. This section also includes a discussion on the spectral differences between the two quasar images. Mass models and the estimate of the Hubble constant are discussed in Sect. 6. Finally, Sect. 7 summarises the main results.