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Extensive X-ray and extreme ultraviolet (EUV) photometric observations of theeclipsing RS CVn system AR Lac were obtained over the years 1997 to 2013 withthe Chandra X-ray Observatory Extreme Ultraviolet Explorer. During primaryeclipse, HRC count rates decrease by ~40%. A similar minimum is seen during oneprimary eclipse observed by EUVE but not in others owing to intrinsic sourcevariability. Little evidence for secondary eclipses is present in either theX-ray or EUV data, reminiscent of earlier X-ray and EUV observations. Primaryeclipses allow us to estimate the extent of a spherically symmetric corona onthe primary G star of about 1.3Rsun, or 0.86Rstar, and indicate the G star islikely brighter than the K component by a factor of 2-5. Brightness changes notattributable to eclipses appear to be dominated by stochastic variability andare generally non-repeating. X-ray and EUV light curves cannot therefore bereliably used to reconstruct the spatial distribution of emission assuming onlyeclipses and rotational modulation are at work. Moderate flaring is observed,where count rates increase by up to a factor of three above quiescence.Combined with older ASCA, Einstein, EXOSAT, ROSAT and Beppo-SAX observations,the data show that the level of quiescent coronal emission at X-ray wavelengthshas remained remarkably constant over 33 years, with no sign of variation dueto magnetic cycles. Variations in base level X-ray emission seen by Chandraover 13 years are only ~10%, while variations back to pioneering Einsteinobservations in 1980 amount to a maximum of 45% and more typically about 15%.

A 33 year constancy of the X-ray coronae of AR Lac and eclipse diagnosis of scale height