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tokens: In the second approach, as a consistency check, we performed an unconstrained fit by allowing for the bulge age to vary between 16.4 and 18.0 Gyr (on steps of 0.2 Gyr) and bulge iron abundance to vary between 0.10 and 0.16 dex (on steps of 0.02 dex) to account for the adopted uncertainties on the bulge properties. For the NSD, we explored the same ranges of age and iron abundance as for the constrained fit. The resulting best-fitting model requires a bulge with t bulge = 18.0 Gyr and [Fe/H] bulge = 0.16 dex and an NSD with t NSD = 3.2 Gyr and [Fe/H] NSD = 0.50 dex (Fig. 5 ). In both cases, we conclude that the NSD of NGC 1023 is much younger and more metal rich than the host bulge. The age of the NSD is well constrained, whereas the its iron abundance corresponds to the upper limit of the [Fe/H] range of the available SSP models. Without the knowledge of the light fraction of NSD, the age and metallicity of the NSD can be hardly recovered due to the degeneracy between these two parameters (see Sarzi et al. 2016 , for a discussion). 4 DISCUSSION AND CONCLUSIONS The structure and properties of the stellar population of the nuclear region of the interacting SB0 galaxy NGC 1023 were investigated through a detailed analysis of archival HST imaging and ground-based integral-field spectroscopy. The galaxy is known to host a highly inclined NSD (Faber et al. 1997 ; Sil'chenko 1999 ; Sarzi et al. 2006 ; Ledo et al. 2010 ). Moreover, the stars of the galaxy nucleus have significantly different chemical properties with respect to the surrounding bulge. They are younger, more metal rich, and have a higher magnesium overabundance (Sil'chenko 1999 ; McDermid et al. 2006 ; Kuntschner et al. 2010 ). In this paper, we derived the central surface brightness, scalelength, inclination, and position angle of the NSD of NGC 1023 in all the available broad-band HST images by assuming that it is an infinitesimally thin exponential disc. We applied the photometric decomposition method of Scorza Bender ( 1995 ) and adopted the algorithm by Morelli et al. ( 2004 , 2010 ), which we implemented for a better determination of the best-fitting parameters. The location and orientation of the NSD do not depend on the observed passband, as already observed for the few other NSDs for which a detailed multiband photometric analysis was performed (Krajnović Jaffe 2004 ; Morelli et al. 2010 ; Corsini et al. 2012 ) whereas the NSD appears more concentrated in the bluer image. We measured the light contribution of the NSD to the total luminosity of the galaxy L NSD / L T in the same concentric circular rings centred on the galaxy nucleus where the equivalent widths of the Lick/IDS line-strength indices were obtained from ground-based integral-field spectroscopy by Sil'chenko ( 1999 ). In the WFPC2/ F555W passband the NSD contributes a maximum of 6 per cent of the total luminosity within the innermost circular aperture ( R 0.65 arcsec), whereas all the galaxy light is given by the