The 2 : 3 period ratio in the light curve of EX Hya.

It is shown that the photometric variation in EX Hydrae with a period slightly longer than two-thirds of the orbital period can be explained by variability in the mass transfer rate caused by pulsation of the lobe-filling secondary. It is argued that the pulsation is a toroidal mode which can be excited by a parametric instability if the orbit is slightly eccentric.