Large-amplitude periodic variations in the angular diameter of R Leonis

We report the first direct detection of long-term periodic diameter variations in a Mira variable. Angular diameter measurements of the 313-d period variable R Leonis at 833 nm and 940 nm obtained between 1996 February and 1997 June using the Cambridge Optical Aperture Synthesis Telescope (COAST) and the William Herschel Telescope (WHT) show a cyclic modulation of the apparent stellar diameter by approximately 35 per cent. The agreement between these new data and archival measurements from 1992 January suggests coherence in the modulation over a 5-yr period, Our data are consistent with recent models which suggest that, in photometric bands with only weak to moderate molecular contamination, periodic variations in stellar diameter of order 50 per cent can be maintained, The measurements indicate that the apparent stellar diameter was largest at visual phase 0.5 and that any phase shifts between the visual light curve and those at 833 and 940 nm were at most 0.05, The large offset (similar to 0.25) between the phase of the observed diameter maximum and that predicted for the photospheric continuum diameter variations suggests that our observations are more sensitive to the changing temperature structure of the outer atmosphere than to the deeper continuum-forming layers.