The optical counterparts of SMC X-2 and SMC X-3.

The spectra of the Sanduleak-Philip optical candidates of SMC X-2 and SMC X-3 are remarkably similar to each other and to the optical X-ray counterparts X Per, Gamma Cas, and HDE 245770. Although it is probable that they are the optical counterparts of SMC X-2 and X-3, their variable X-ray luminosity, when detected, was considerably higher than that of optically similar variable sources in the Galaxy. He II 4686-A emission is present, although very weak, in both spectra and appears to have large velocity and intensity changes on a time scale of days.