EXOSAT OBSERVATIONS OF THE ULTRA-SOFT X-RAY BINARY 4U-1957+11

We present results from the analysis of the two EXOSAT observations of the low mass X-ray binary (LMXRB) 4U 1957+11. The 1-20 keV spectrum o f the source is best fitted by a power law model with exponential cuto ff; that provides an approximation of a thermal Comptonisation spectru m. The cutoff energy (similar to 2 keV), as well as the X-ray colours, are intermediate between those of ultrasoft sources containing black hole candidates (BHCs) and those of soft LMXRBs containing an old accr eting neutron star. We find no evidence for a black body spectral comp onent. During the 1985 observation the source flux was similar to 10% higher than in the 1983 observation, and the ME spectra provide eviden ce for a high energy tail (up to similar to 16 keV), which strengthens the similarity with the spectra of high state BHCs. However the sourc e luminosity was only similar to 5 x 10(36)ergs s(-1). During the 1985 observation the GSPC spectra revealed the presence of a 100 eV EW iro n K-line emission feature with a centroid energy of 7.06 +/- 0.09 keV; this value is among the highest measured by EXOSAT. The ME light curv es showed only a low-amplitude variability on timescales of a few hour s. Mo evidence for a periodic modulation with periods between 0.016 an d 13000 s was found.