  We present near-infrared K-band interferometric measurements of the Mira star
R Leonis obtained in April 2001 and January 2002 with the VLTI, the
commissioning instrument VINCI, and the two test siderostats. These epochs
correspond to near-maximum stellar variability phases $\sim$ 0.08 and $\sim$
1.02 (one cycle later), respectively. The April 2001 data cover a range of
spatial frequencies (31--35 cycles/arcsecond) within the first lobe of the
visibility function. These measurements indicate a center-to-limb intensity
variation (CLV) that is clearly different from a uniform disk (UD) intensity
profile. We show that these measured visibility values are consistent with
predictions from recent self-excited dynamic Mira model atmospheres that
include molecular shells close to continuum-forming layers. We derive
high-precision Rosseland diameters of 28.5 +/- 0.4 mas and 26.2 +/- 0.8 mas for
the April 2001 and January 2002 data, respectively. Together with literature
estimates of the distance and the bolometric flux, these values correspond to
linear radii of 350^{+50}_{-40}R$_\odot$ and 320^{+50}_{-40}R$_\odot$, and to
effective temperatures of 2930 +/- 270K and 3080 +/- 310K, respectively.

Astrophysics