Interferometric observations of R Leonis in the K band - First direct detection of the photospheric pulsation and study of the atmospheric intensity distribution

The Mira-type star R Leonis was observed at two different epochs in the K b and with the FLUOR beam combiner on the IOTA interferometer. A variation of diameter is clearly detected for the very first time at this wavelength re vealing an apparent pulsation of atmospheric layers very close to the stell ar photosphere. We discuss the excess of visibility measured at high spatia l frequencies and show that they very likely reveal smaller diameters of th e photosphere (22-24 mas) than those deduced on the basis of shorter freque ncy components. This smaller diameter makes R Leonis a fundamental pulsator . Mira models are compared to our data and a disagreement with spatial inte nsity distribution and dynamical behaviour is found.