Infrared spectroscopy of the Be/X-ray transient A0535+26

We present infrared spectroscopy of the Be/X-ray binary HDE 245770/A0535 + 26 obtained over the period 1992-1995. The spectra show significant variability, reflecting changes in the circumstellar environment during this time. A reduction in the flux observed in the Paschen series lines between 1993 December and 1994 September correlates with a similar reduction in both the strength of Ha and the optical continuum emission, which can be attributed to a reduction in the emission measure of the disc. A turnover between optically thin and thick emission is seen for both Paschen and Brackett series lines, and allows an estimate of the disc density as similar to 10(12) cm(-3). Echelle spectroscopy reveals strong similarities between the He I 1.008, 2.058 mu m, H alpha and Paschen series line profiles, suggesting their formation in a similar (and asymmetric) region of the disc. In contrast, the line profile of He I 6678 Angstrom indicates that it is formed at smaller radii than the other transitions.