The 1991 24 hr Ultraviolet Monitoring of 21 Com with IUE

The comparison of 10 far ultraviolet spectra of 21 Com taken over continuous 24 hr (half of the recently determined rotational period) reveals large variations shortwards of 1700 Å. In contrast, the flux variations at wavelengths larger then 2200 Å are of smaller amplitudes. The large FUV variability is probably caused by changes in the continuous opacity driven by modest horizontal gradients of the abundances of chemical elements over the surface of 21 Com.