Checking of the hydrostatic and spherical symmetric approximation for the interferometric data. The series of panels “a” refers to DR Ser data, panels “b” to HK Lyr, and panels “c” to RV Mon. The plots in Col. “1” represents the uv-coverage of the observations, in Col. “2” the visibility points acquired are plotted versus the baselines. In Col. “3” the correspondent UD diameter is plotted versus position angle. In Col. “4” the UD diameter is plotted versus the visual phase.