  The observed radii of M dwarfs in eclipsing binaries exceed predicted radii
by ~ 5%. To investigate this anomaly, the structure of low-mass stars
irradiated by a close companion is considered. Irradiation modifies the surface
boundary conditions and thereby also the adiabatic constants of their outer
convection zones. This changes the models' radii and luminosities. For
short-period M dwarf binaries with components of similar mass, radius inflation
due to their mutual irradiation is found to be < 0.4%. This is an order of
magnitude too small to explain the observed anomaly. Although stronger
irradiation results in a monotonically increasing radius, a saturation effect
limits the inflation to <~ 5\%.
