1 Introduction

High-resolution interferometric observations of Mira stars allow the study of the size of the stellar disk, photospheric asymmetries, surface inhomogenities, and the wavelength, pulsation phase and pulsation cycle dependence of the diameter (see, e.g., Pease 1931; Bonneau & Labeyrie 1973; Labeyrie et al. 1977; Bonneau et al. 1982; Karovska et al. 1991; Haniff et al. 1992; Quirrenbach et al. 1992; Wilson et al. 1992; Tuthill et al. 1994; Danchi et al. 1994; Haniff et al. 1995; Hofmann et al. 1995a; Weigelt et al. 1996; van Belle et al. 1996; Burns et al. 1998; Perrin et al. 1999; Hofmann et al. 2000a; Hofmann et al. 2000b; Weigelt et al. 2000). Theoretical studies (e.g. Watanabe & Kodaira 1979; Scholz 1985; Bessell et al. 1989; Bessell et al. 1996 = BSW96) show that accurate monochromatic diameter measurements can improve our understanding of M giant atmospheres. Previous interferometric single-dish observations of R Leo at optical wavelengths were reported by Labeyrie et al. (1977), Tuthill et al. (1994, 1999) and Haniff et al. (1995). Di Giacomo et al. (1991) reported high-resolution lunar occultation observations of R Leo in the near infrared, Danchi et al. (1994) interferometric 11.5 $\mu$m measurements, Burns et al. (1998) observations at optical wavelengths with the COAST interferometer, and Perrin et al. (1999) K-band observations with the IOTA interferometer.

In this paper we present diffraction-limited 2-dimensional images of R Leo reconstructed by the bispectrum speckle interferometry method and comparisons of the observations with Mira star models.