High-amplitude, long-term X-ray variability in the solar-type star HD 81809: The beginning of an X-ray activity cycle?F. Favata, G. Micela, S. L. Baliunas, J. H. M. M. Schmitt, M. Güdel, F. R. Harnden, S. Sciortino and R. A. SternA&A, 418 1 (2004) L13-L16DOI: https://doi.org/10.1051/0004-6361:20040105
