Context : X-ray observations performed with the Röntgen Satellite ( ROSAT ) led to the discovery of a group ( seven to date ) of X-ray dim and radio-silent middle-aged isolated neutron stars ( a.k.a .
XDINSs ) , which are characterised by pure blackbody spectra ( kT \approx 40 - 100 eV ) , long X-ray pulsations ( P = 3 - 12 s ) , and appear to be endowed with relatively high magnetic fields , ( B \approx 10 ^ { 13 } – 10 ^ { 14 } G ) .
Optical observations of XDINSs are important , together with the X-ray ones , to study the cooling of the neutron star surface and to investigate the relation betwen XDINSs and other isolated neutron star classes .
RBS 1774 is one of the few XDINSs with a candidate optical counterpart , which we discovered with the Very Large Telescope ( VLT ) .

Aims : We aim at constraining the optical spectrum of RBS 1774 , for which only two B-band flux measurements are available , and to determine whether its optical emission is either of thermal or of non-thermal origin .

Methods : We performed deep observations of RBS 1774 in the R band with the VLT to disentangle a non-thermal power-law spectrum from a Rayleigh-Jeans , whose contributions are expected to be very much different in the red part of the spectrum .

Results : We did not detect the RBS 1774 candidate counterpart down to a 3 \sigma limiting magnitude of R \sim 27 .
The constraint on its colour , ( B - R ) \la 0.6 , rules out that it is a background object , positionally coincident with the X-ray source .
Our R-band upper limit is consistent with the extrapolation of the B-band flux ( assuming a 3 \sigma uncertainty ) for a set of power-laws F _ { \nu } \propto \nu ^ { - \alpha } with spectral indeces \alpha \leq 0.07 .
If the optical spectrum of RBS 1774 were non-thermal , its power-law slope would be very much unlike those of all isolated neutron stars with non-thermal optical emission , suggesting that it is most likely thermal .
For instance , a Rayleigh-Jeans with temperature T _ { O } = 11 eV , for an optically emitting radius r _ { O } = 15 km and a source distance d = 150 pc , would be consistent with the optical measurements .
The implied low distance is compatible with the 0.04 X-ray pulsed fraction if either the star spin axis is nearly aligned with the magnetic axis or with the line of sight , or it is slightly misaligned with respect to both the magnetic axis and the line of sight by 5 - 10 ^ { \circ } .

Conclusions : New observations , both from the ground and from the Hubble Space Telescope ( HST ) , are important to characterise the optical/near-ultraviolet ( UV ) spectrum of RBS 1774 , to better constrain the values of r _ { O } , d , and T _ { O } and measure the source proper motion from which indirect constraints on the source distance can be inferred .