HUBBLE-SPACE-TELESCOPE IMAGING OF BRIGHT GALACTIC-X-RAY BINARIES IN CROWDED FIELDS

We report high spatial resolution HST imagery and photometry of three well-studied, intense Galactic X-ray binaries, X2129+470, CAL 87, and GX 17+2. All three sources exhibit important anomalies that are not re adily interpreted by conventional models. Each source also lies in a s everely crowded held, and in all cases the anomalies would be removed if much of the light observed from the ground in fact came from a near by, thus far unresolved superposed companion. For V1727 Cyg (X2129+470 ), we find no such companion. We also present an HST FOS spectrum and broadband photometry which is consistent with a single, normal star. T he supersoft LMC X-ray source CAL 87 was already known from ground-bas ed work to have a companion separated by 0''.9 from the optical counte rpart; our HST images clearly resolve these objects and yield the disc overy of an even closer, somewhat fainter additional companion. Our ph otometry indicates that contamination is not severe outside eclipse, w here the companions only contribute 20% of the light in V, but during eclipse more than half of the V light comes from the companions. The p reviously determined spectral type of the CAL 87 secondary may need to be reevaluated due to this significant contamination, with consequenc es on inferences of the mass of the components. We find no companions to NP Ser (=X1813-14, = GX 17+2). However, for this object we point ou t a small but possibly significant astrometric discrepancy between the position of the optical object and that of the radio source which is the basis for the identification. This discrepancy needs to be clarifi ed.