  We show that in evolved stars, even at relatively low surface amplitudes,
nonradial modes become strongly nonlinear in the hydrogen shell source, where
the Brunt-Vaisala frequency has its absolute maximum. The measure of
nonlinearity is the product of horizontal displacement times the radial
wavenumber. It becomes large already in evolved delta-Scuti stars. This
nonlinearity presents a major problem for interpretations of amplitude
modulation in RR Lyrae stars in terms of nonradial mode excitation.
