We present results on the variability of the Balmer line profiles in two very
different stars, an ultra-fast rotator, FK Comae, and a classical T Tauri star,
SU Aurigae.
The data were obtained during the 96 MUSICOS campaign and also with the MUSICOS
spectrograph at the INT in May 97.
FK Comae is a G5 II solar-type active giant. It has an extreme rotational velocity near break up. This star presents a very broad and variable Halpha profile. We analyse these profiles as arising from emission from loop-like structures extending to several stellar radii, anchored in active regions on the stellar photosphere.
SU Aur is a classical T Tauri star that shows little veiling and few emission lines. The Balmer line profiles are very variable, even in short time scales. We present results on a preliminary modelling of this line, based on this new data set, attempting to describe the interaction of the stellar magnetosphere and the accretion disc.