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 Leonis obtained by lunar occultations with broad- and narrowband filters at 2.2 and 3.36 μm, respectively. Observations were taken at the visual variability phases of 0.44 and 0.17 of the same pulsational cycle. We derive a uniform-disk (UD) diameter at 3.36 μm of 39 ± 3 mas, in good agreement with optical interferometric measurements at the same phase of an earlier cycle. From the 3.36 μm observations we derive an effective temperature of 2257 ± 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 μm. The 2.2 μm observations yield a physical diameter that is significantly lower (at least 20%) than the averaged optical interferometric values measured at nearly the same phase of the previous cycle. In combination with the 3.36 μm determination, this result implies that the variation in angular diameter of R Leo with phase at IR wavelengths is small (<5%), in contrast to what is observed in the optical. The linear radii derived from both the 3.36 and 2.2 μm observations seem to favor the first-overtone mode of pulsation, rather than the fundamental one.