c © ESO 2003 Astronomy &Astrophysics JHK′-band IOTA interferometry of the circumstellar environment

Abstract. We report the first long-baseline interferometry of the circumstellar dust environment of R CrB. The observations were carried out with the Infrared Optical Telescope Array (IOTA), using our new JHK0 beam combiner which enables us to record fringes in the J, H, and K0 bands simultaneously. The circumstellar dust envelope of R CrB is resolved at a baseline of 21 m along a position angle of 170, and the visibilities in the J, H, and K0 bands are 0.970.06, 0.780.06, and 0.610.03, respectively. These observed visibilities, together with the K0-band visibility obtained by speckle interferometry with baselines of up to 6 m, and the spectral energy distribution are compared with predictions from spherical dust shell models which consist of the central star and an optically thin dust shell. The comparison reveals that the observed J- and H-band visibilities are in agreement with those predicted by these models, and the inner radius and inner boundary temperature of the dust shell were derived to be 60–80 R? and 950–1050 K, respectively. However, the predicted K0-band visibilities are found to be 10 % smaller than the one obtained with IOTA. Given the simplifications adopted in our models and the complex nature of the object, this ca