EUVE photometric eclipse observations of AR Lacertae

We report on observations of the eclipsing RS CVn system AR Lac with the Extreme Ultraviolet Explorer (EUVE) scanning and deep survey (DS) telescopes. This system consists of a G2 IV star and a KO IV star separated by 9.2R. with an orbital period of about 1.98 days. The scanner observations consisted of a single pointing over a period of 25 hours, providing some coverage of both primary and secondary eclipses, and subsequent scanning during the all-sky survey. During the pointed observation AR Lac was detected in the EUVE Lexan/boron (Lexan/B, 50-180 Angstrom) and Al/Ti/C (160-240 Angstrom) bands at average count rates of 0.12 and 0.016 count s(-1), respectively. During primary eclipse, the Lexan/B count rate was at about one-third of its subsequent value at quadrature, suggesting that, in the case of a compact and relatively homogeneous corona, a minimum of 60%-70% of the extreme ultraviolet (EUV) emission originates on the G star. There was no detectable modulation corresponding to secondary eclipse in either Lexan/B or Al/Ti/C bands. Data taken several months later during the EWE all-sky survey yield the same Lexan/B count rate as the pointed data but only an upper limit in the Al/Ti/C bandpass. AR Lac was observed again with the DS as part of a Guest Observer pointed observation. This single pointing lasted slightly more than three days and included both primary and secondary eclipses. AR Lac was detected in the DS Lexan/B band with an average count rate of 0.31 count s(-1). The primary eclipse is easily visible as a similar to 40% decrease in intensity. A dip in the light curve is present at the time of secondary eclipse, but is not readily discernible from the average level of variability. A flare corresponding to a factor of 2 increase over the quiescent emission count rate was observed during the next expected secondary eclipse. We have performed an elementary emission measure analysis and compared isothermal models that fit the EUVE data to earlier Einstein, EXOSAT, ASCA, and ROSAT two-temperature models culled from the literature. In general, the EUVE data indicate very similar emission measures to those derived in previous work, indicating that the corona of AR Lac does not vary in average x-ray or EW luminosity over periods of years up to a decade. Assuming an isothermal model with T approximate to 10(7) K, as indicated by the emission measure analysis, the total Lexan/B count rate at quadrature implies an EUV luminosity of 1.1 x 10(30) erg s(-1) in the 50-180 Angstrom bandpass. (C) 1996 American Astronomical Society.