Hubble Space TelescopeImaging of Bright Galactic X‐Ray Binaries in Crowded Fields

We report high spatial resolution HST imagery and photometry of threewell-studied, intense galactic X-ray binaries, X2129+47, CAL 87 and GX17+2. Allthree sources exhibit important anomalies, not readily interpreted byconventional models. Each source also lies in a severely crowded field, and inall cases the anomalies would be removed if much of the light observed from theground in fact came from a nearby, thus-far-unresolved superposed companion.For V1727 Cyg (X2129+470), we find no such companion. We also present an HSTFOS spectrum and broadband photometry which is consistent with a single, normalstar. The supersoft LMC X-ray source CAL 87 was already known from ground-basedwork to have a companion separated by 0.9'' from the optical counterpart; ourHST images clearly resolve these objects, and yield the discovery of an evencloser, somewhat fainter additional companion. Our photometry indicates thatcontamination is not severe outside eclipse, where the companions onlycontribute 20% of the light in V, but during eclipse more than half of the Vlight comes from the companions. The previously-determined spectral type of theCAL 87 secondary may need to be reevaluated due to this significantcontamination, with consequences on inferences of the mass of the components.We find no companions to NP Ser (=X1813-14, =GX17+2). However, for this objectwe point out a small but possibly significant astrometric discrepancy betweenthe position of the optical object and that of the radio source which is thebasis for the identification. This discrepancy needs to be clarified.