The linear polarization of the 1s2p 1 P 1 →1s 2 1 S 0 resonance line, the 1s2p 3 P 1,2 →1s 2 1 S 0 intercombination lines, and the 1s2s 3 S 1 →1s 2 1 S 0 forbidden line was measured in heliumlike Fe XXV excited near threshold by a monoenergetic electron beam. The measurement was carried out with a high-resolution x-ray spectrometer employing a set of two analyzing crystals that acted as polarizers by selectively reflecting the individual polarization components. A value of ϩ0.56 Ϫ0.08 ϩ0.17 was determined for the polarization of the 1 P 1 line, Ϫ0.53 Ϫ0.02 ϩ0.05 for the 3 P 2 line, Ϫ0.22 Ϫ0.02 ϩ0.05 for the 3 P 1 line, and Ϫ0.076 Ϫ0.007 ϩ0.007 for the 3 S 1 line. The measurements were compared with results from a relativistic distorted-wave calculation, which was carried out for a number of mid-Z heliumlike ions ͑Mg 10ϩ-Kr 34ϩ ͒, and good agreement was found. By contrast, disagreement was noted with predictions based on Coulomb-Born calculations, allowing us to distinguish between theoretical approaches. ͓S1050-2947͑96͒05406-6͔
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