  Multiwavelength studies of the seven identified X-ray dim isolated neutron
stars (XDINSs) offer a unique opportunity to investigate their surface thermal
and magnetic structure and the matter-radiation interaction in presence of
strong gravitational and magnetic fields. As a part of an ongoing campaign
aimed at a complete identification and spectral characterization of XDINSs in
the optical band, we performed deep imaging with the ESO Very Large Telescope
(VLT) of the field of the XDINS RBS1774 (1RXS J214303.7 +065419). The recently
upgraded FORS1 instrument mounted on the VLT provided the very first detection
of a candidate optical counterpart in the B band. The identification is based
on a very good positional coincidence with the X-ray source (chance probability
~2E-3). The source has B=27.4 +/- 0.2 (1 sigma confidence level), and the
optical flux exceeds the extrapolation of the X-ray blackbody at optical
wavelengths by a factor ~35 (+/- 20 at 3sigma confidence level). This is barely
compatible with thermal emission from the neutron star surface, unless the
source distance is d~200-300 pc, and the star is an almost aligned rotator or
its spin axis is nearly aligned with the line of sight. At the same time, such
a large optical excess appears difficult to reconcile with rotation-powered
magnetospheric emission, unless the source has an extremely large optical
emission efficiency. The implications and possible similarities with the
optical spectra of other isolated NSs are discussed.
