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K-band spectroscopy of Be-star X-ray binaries

We present the first 2.0-2.5 μm spectra of four Be/X-ray binaries, at a resolution of λ/Δλ = 310-400. The spectra show strong hydrogen recombination lines, notably Brγ and, in most cases, the higher order Pfund series. The relative recombination strengths imply that Brγ has a large optical depth, whereas the Pfund lines have relative strengths close to case B and so may be optically thin. The He I 2.06-μm line is also in emission. Mg II emission lines are present, as are Na I lines; the latter are normally associated with emission-zone temperatures that are cooler than the conventional Be-star circumstellar disc. We conclude that optical effects are evident in H I line emission