Shape and location of the sonic surface near the inner Lagrangian point. The 3D images of the binary near the Lagrangian point in the center show the orientation of the 2D plane in which the 2D distribution of Mach number is depicted – mid- (top) and vertical (bottom) slices. The panels on the left show the distributions for the equal-mass adiabatic case, while those on the right correspond to the equal-mass isothermal case. In the panels showing the Mach number distribution, the dashed diagonal orange lines depict the shape of the Roche lobe.