(the following information provided by Dr. Pierre Chayer)
Drs. Damian Christian, and Stuart Bowyer of CEA, Dr. Peter Vedder of Omitron, Inc., Dr. Jeremy Drake of the Harvard-Smithsonian Center for Astrophysics, and Dr. Robert Patterer of the Goddard Space Flight Center (GSFC) reported on EUVE photometric eclipse observations of AR Lacertae.
AR Lacertae is a totally eclipsing RS CVn binary composed of a G2IV and a K0IV star separated by a distance of about 9.2 solar radius. The orbital period of this system is 1.983 days. Because the orbital separation is small, tidal viscosity tends to synchronize the stellar rotation and orbital periods, increasing the rotation rates beyond typical values for single late-type stars. This rapid rotation makes these stars strongly magnetically active and, therefore, strong sources of chromospheric and coronal emission.
AR Lacertae was observed with the EUVE scanning and deep survey telescopes. The observations cover approximately two orbital periods, including both primary and secondary eclipses. The deep survey observation detected a flare at the time of expected secondary eclipse. The deep survey and scanner observations show a 40% to 60% reduction in flux, respectively, when the G star is eclipsed by the K star. There was no discernible decrease in count rate during the secondary eclipse (i.e., the K star eclipsed by the G star), which supports the idea previously put forth that the K star has an extended corona.
The qualitative similarities between the Lexan/B deep survey and scanner light curves and those derived from observations made with EXOSAT and the ROSAT PSPC suggest that either these coronal features are grossly stable over a time scale of years or else the similarities in the light curves are purely coincidental.