indicate that AGN, if present, are weak. From the spectral decomposition made possible by the combination of the available data over a wide frequency range, Negrello et al. (2014, see their figure 6) evaluate the AGN bolometric luminosities $L_{\rm AGN}\sim5.1\times 10^{45} {\rm erg/s}$ for SDP.9 and $L_{\rm AGN}\sim4.3\times 10^{46} {\rm erg/s}$ for SDP.11, although these values may be overestimated as they have been evaluated in a poorly sampled spectral region where contamination might enhance the signal in an unpredictable way. For these reasons we consider these values as a pure indication for the presence of an AGN and as loose upper limits of its luminosity.

In the near-IR SDP.9 consists of a dominant emitting region of $\sim1$ kpc in size, and a smaller and fainter one separated by a few kpc responsible for the fainter structure observed in the $\sim0.71$ arcsec radius Einstein ring (Dye et al. 2014). By the SED fitting, Negrello et al. (2014) estimated that the background source has a mass in stars $M\star \sim 7.1 \times 10^{10} M_{\sun}$. Lupu et al. (2012) estimated a comparable mass in molecular gas, $M_{\rm gas} \sim 3.4 \times 10^{10} M_{\sun}$ on the basis of CO emission. The gas fraction is 