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Magnetic fields play a role in the heating of coronae in the Sun and cool dwarfs, but the physical processes at work are not yet clear. Several magnetic models have been proposed for coronal heating. In one class of models, resonant absorption of MHD waves in closed loops is believed to be at work. In a very general sense, resonance implies the existence of a preferred frequency. Thus, resonant absorption models would be subject to a decisive test if periodicities could be identified in the coronal emission. We have searched for periodicities in the EINSTEIN X-ray fluxes of 15 cool dwarfs. Statistically significant periodicities have been detected in several stars, including EQ Vir, AD Leo, and Prox Cen. The period detected in Prox Cen is consistent with MHD resonance in the loop for which parameters were derived by Haisch in his study of X-ray light curves in Einstein data. For AD Leo, ROSAT/HRI and EXOSAT data also suggest the presence of periodicity at the same period as in the EINSTEIN data. For EQ Vir, EXOSAT data suggest the presence of the same period as in the EINSTEIN data. However, the statistical significance of the ROSAT and EXOSAT periods is not as good as those for EINSTEIN.
\vskip0.2truein \noindent This work has been supported by NASA Grant NAGW-2456, by the NASA Space Grant College program, and by the NASA Astrophysical Data Program.
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