Average diameter θ vs. observing time after the first epoch (2017-09-30) for the star R Leo. The black circles indicate the radius at 140 GHz in Band 4 and the red squares are the fitted radii at 225 GHz in Band 6. The black solid line is the error weighted fitted expansion of the optically thick surface. The expansion velocity corresponds to 10.6 ± 1.4 km s⁻¹ (assuming a distance of 71 pc). The red dashed line is the fit to the 140 GHz radii shifted by a time lag of −5 days.