On the lensed blazar B0218+357
  We present an optical spectrum (4000-10500 {\AA} ) of the lensed blazar
B0218+357 secured at the 10m GTC and aimed to investigate and clarify the
properties of this intriguing system. We found that the emission line spectrum
of the blazar is characterised by only one broad emission line that interpreted
as Mg II 2800 {\AA} yields z=0.95. In addition we detect narrow absorption
lines of Mg II 2800 {\AA} and Ca II (H,K) and Na I 5892 {\AA} at z=0.68437 +/-
0.00005 due to intervening interstellar gas. No stellar absorption features
attributable to the lens galaxy are revealed. Thus the assumed redshift of the
lens is dubious. The continuum spectrum of the object exhibits a remarkable
drop towards the short wavelengths likely due to a significant extinction. This
extinction cannot be produced in the lens galaxy at z =0.684 with any value of
R$_V$ under the assumption that the intrinsic shape of the blazar is dominated
by a power law emission. However, the observed continuum is consistent with a
power law emission assuming a standard (R$_V$ = 3.1) extinction at the source
redshift (z=0.95) as supported also by the presence of Mg II absorptions at the
same redshift. HST images of B0218+357 exhibit the double image of the source
together with extended image of a face on spiral galaxy. We argue that this
galaxy is possibly not the lensing galaxy but the host galaxy of the blazar.
This has substantial consequences on the models of the system and on the
derived values of the Hubble constant.
