The angular diameter of the Mira variable R Leonis at 3.36 and 2.2 microns

We report angular diameter determinations of the bright Mira variable R Leo nis obtained by lunar occultations with broad- and narrowband filters at 2. 2 and 3.36 mu m, respectively. Observations were taken at the visual variab ility phases of 0.44 and 0.17 of the same pulsational cycle. We derive a un iform-disk (UD) diameter at 3.36 mu m of 39 +/- 3 mas, in good agreement wi th optical interferometric measurements at the same phase of an earlier cyc le. From the 3.36 mu m observations we derive an effective temperature of 2 257 +/- 110 K and a linear radius value of 478 +/- 50 R. using an accurate value of parallax. A UD value of 34 +/- 2 mas is obtained at 2.2 mu m. The 2.2 mu m observations yield a physical diameter that is significantly lower (at least 20%) than the averaged optical interferometric values measured a t nearly the same phase of the previous cycle. In combination with the 3.36 mu m determination, this result implies that the variation in angular diam eter of R Leo with phase at IR wavelengths is small (<5%), in contrast to w hat is observed in the optical. The linear radii derived from both the 3.36 and 2.2 mu m observations seem to favor the first-overtone mode of pulsati on, rather than the fundamental one.