Abstract
Chromospheric-coronal emission of cool stars has a clear observed (spectral type dependent) upper limit (saturation). This boundary is seen in the surface flux vs. color (and flux vs. period) diagrams for different types of cool stars, excluding T Tauri-stars. The limit is occupied by rapid rotators (τ c /P≥3), either very young stars or components in close binaries with tidally forced rotation. Saturated stars seem to be almost totally filled with equipartition magnetic fields. This is one of the reason for the saturation: super-active regions with B= 1–2 kG fill the stellar surface, and no more magnetic flux can get out. This is due to the feed-backs between magnetic fields, convection and differential rotation.
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