Multiwavelength diameters of nearby Miras and semiregular variables

We have used optical interferometry to obtain multiwavelength visibility curves for eight red giants over the wavelength range 650-1000 nm. The observations consist of wavelength-dispersed fringes recorded with MAPPIT (Masked Aperture-Plane Interference Telescope) at the 3.9-m Anglo-Australian Telescope. We present results for four Miras (R Car, o Cet, R Hya and R Leo) and four semiregular variables (R Dor, W Hya, L-2 Pup and gamma Cru). All stars except gamma Cru show strong variations of angular size with wavelength. A uniform-disc model was found to be a poor fit in most cases, with Gaussian (or other more tapered) profiles preferred. This, together with the fact that most stars showed a systematic increase in apparent size towards the blue and a larger-than-expected linear size, even in the red, all point towards significant scattering by dust in the inner circumstellar environment. Some stars showed evidence for asymmetric brightness profiles, while L-2 Pup required a two-component model, indicating an asymmetrical circumstellar dust shell.