Cyclic variations in the angular diameter of chi Cygni

We report the direct detection of cyclic diameter variations in the Mira va riable chi Cygni. Interferometric observations made between 1997 July and 1 999 September, using the Cambridge Optical Aperture Synthesis Telescope (CO AST) and the William Herschel Telescope (WHT), indicate periodic changes in the apparent angular diameter at a wavelength of 905 nm, with amplitude 45 per cent of the smallest value. The star appears largest at minimum light. Measurements made at a wavelength of 1.3 mum over the same period suggest much smaller size changes. This behaviour is consistent with a model in whi ch most of the apparent diameter variation at 905 nm is caused by a large i ncrease in the opacity of the outer atmospheric layers (which is mostly owi ng to titanium oxide) near minimum light, rather than by physical motions o f the photosphere. The 1.3-mum waveband is relatively uncontaminated by TiO , and so much smaller size changes would be expected in this band. The late st non-linear pulsational models predict maximum physical size close to max imum Light, and increases in opacity near minimum light that are too small to reproduce the diameter variation seen at 905 nm. This suggests either th at the phase-dependence of the model pulsation is incorrect, or that the op acities in the models are underestimated. Future interferometric monitoring in uncontaminated near-infrared wavebands should resolve this question.