LARGE-AMPLITUDE PERIODIC VARIATIONS IN THE ANGULAR DIAMETER OF R-LEONIS

We report the first direct detection of long-term periodic diameter va riations 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 cycl ic 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 per iod, 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 m aintained, The measurements indicate that the apparent stellar diamete r was largest at visual phase 0.5 and that any phase shifts between th e visual light curve and those at 833 and 940 nm were at most 0.05, Th e large offset (similar to 0.25) between the phase of the observed dia meter maximum and that predicted for the photospheric continuum diamet er variations suggests that our observations are more sensitive to the changing temperature structure of the outer atmosphere than to the de eper continuum-forming layers.