2S 1553-542

R. L. Kelley, S. Ayasli and S. Rappaport, Massachusetts Institute of Technology, report the detection of 9.3-s x-ray pulsations from the galactic x-ray source 2S 1553-542 (Apparao et al. 1978, Nature 271, 225). The data were obtained with the SAS-3 rmc detectors in 1975 June. The source had an intensity of 27 uJy (2-11 keV) with a pulsation amplitude of ~ 80 percent. The pulsations, coupled with the object's hard x-ray spectrum and apparently transient nature - it was not detected by Uhuru - are possibly indicative of a Be-star x-ray binary (Rappaport and van den Heuvel 1982, IAU Symposium No. 98, in press). No optical counterpart has yet been identified. Optical observers are therefore urged to carry out spectroscopic observations of the brighter stars in the small x-ray error box of Apparao et al. (loc. Cit.) during the current observing season.