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Precise celestial positions have been obtained with the HEAO 1 scanning modulation collimators for the highly variable X-ray source GX 339--4 (4U 1658--48) and the X-ray burst source MXB 1659--29. Both sources are identified with faint (17-18 mag) blue objects with He II lambda4686 and lambdalambda4640--50 emission.
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Doxsey et al. (1979) studied this question.
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