A Probable Optical Counterpart for the Isolated Neutron Star RX   J1308.6+2127
  Using a very deep observation with HST/STIS, we have searched for an optical counterpart to the nearby radio-quiet isolated neutron star RX J1308.6+2127 (RBS 1223). We have identified a single object in the 90% Chandra error circle that we believe to be the optical counterpart. This object has $m_{50CCD}=28.56\pm0.13$ mag, which translates approximately to an unabsorbed flux of $F_{\lambda}=(1.7 \pm 0.3)e-20$ ergs/s/cm^2/A at 5150 A or an X-ray-to-optical flux ratio of $log(f_X/f_opt)=4.9$. This flux is a factor of $\approx 5$ above the extrapolation of the black-body fit to the X-ray spectrum, consistent with the optical spectra of other isolated neutron stars. Without color information we cannot conclude that this source is indeed the counterpart of RX J1308.6+2127. If not, then the counterpart must have $m_{50CCD} > 29.6$ mag, corresponding to a flux that is barely consistent with the extrapolation of the black-body fit to the X-ray spectrum. 
D. K. Kaplan, S. R. Kulkarni and M. H. van Kerkwijk