Spectroscopic follow-up of double quasar candidates
  We report the results of an optical spectroscopic follow-up of four double
quasar candidates in the Sloan Digital Sky Survey (SDSS) database. SDSS
J1617+3827 is most likely a lensed quasar at z = 2.079, consisting of two
images with r ~ 19-21 and separated by ~ 2 arcsec. We identify an extended
source northeast of the brightest image as an early-type lensing galaxy at z =
0.602, and detect a candidate for the main deflector in the vicinity of the
faintest image. SDSS J2153+2732 consists of two distinct but physically
associated quasars at z ~ 2.24 (r ~ 19-20, separation of 3.6 arcsec). Although
this system might be a binary quasar, there is evidence of a collision or
merger within a galaxy cluster at an early stage. The other two candidates are
projected pairs of active galactic nuclei: SDSS J1642+3200 (separation of ~ 3
arcsec) comprises a distant quasar (r ~ 18) at z = 2.263 and the active nucleus
(r > 20) of a galaxy at z ~ 0.3, while SDSS J0240-0208 (r ~ 18-19, separation
of ~ 1 arcsec) is a pair of quasars at z = 1.687 and z = 1.059. In each of
these two systems, the background quasar only suffers a weak gravitational
lensing effect by the host galaxy of the foreground active nucleus, so the host
galaxy mass is constrained to be less than (2.9-3) x 10^{11} solar masses
inside 10 kpc.
