Interferometric observations of evolved stars in the L′ band

Ten bright Miras and eight semi-regular variable giants and supergiants have been observed with the IOTA (Infrared and Optical Telescope Array) interferometer in the L' band (from 3.4 to 4.1 microns). Observations were carried out in March and November 2000 with the FLUOR/TISIS instrument. Variations in the diameter of R Leo are for the first time observed in the L' band, and our data show that the diameter of alpha Ori is remarkably stable. Important deviations from a uniform disk model are demonstrated for most of the Mira stars of our sample, and are particularly obvious for chiCyg, R Cas and o Cet. Observations of a Her carried out in March are consistent with previous estimates of its diameter published with the very first data of TISIS. The instrument has shown its reliability and the good quality of its data. The present results emphasize the importance of such data for a better comprehension of the circumstellar environment of evolved stars.