1996PASP..108..688D

Among the about one dozen luminous X-ray bursters in the cores of globular clusters, only three have optical/UV counterparts unambiguously identified. We have observed two of these counterparts spectroscopically with the low resolution optical and UV gratings of the Faint Object Spectrograph (FOS) aboard the Hubble Space Telescope (HST), yielding broad (1150-2500A and 3600-7000A) wavelength coverage in each case. For the counterpart AC211 to the X-ray source in NGC 7078/M15, our small aperture FOS data reveal both strong absorption and emission features, as well as a broad spectral energy distribution (SED), that are akin to those determined from earlier ground-based and IUE spectra, but with some important differences. These differences suggest that only the HST spectra of AC211 are free of significant contamination from nearby cluster stars. Our FOS data for the counterpart "Star S" in NGC 6712 permit an investigation of the SED for this second object as well; in this case a composite (and/or variable) SED is indicated. However, intriguingly in the case of "Star S", we find little evidence for any identifiable features–emission or absorption–attributable to the system, although features as strong as those seen for AC211 would be readily detected. These HST optical and UV spectra of AC211 and "Star S" may presage a spectral diversity among the counterparts to X-ray bursters in globular cluster cores.