Confirmatory evidence for changing light-curve amplitude of the former eclipsing and current SB2 system SS Lac in the Open Cluster NGC 7209 has been uncovered. Remeasured Harvard plate data and published and compiled data sets reveal that the depth of the primary minimum increased between the 1890s and 1902 and decreased in the 1920s and 1930s. A parabolic fitting of the amplitude with phase predicts a maximum at 1911.5, with an eclipse onset at 1885.3 and eclipse cessation at 1937.8. We confirm the finding of Lehmann, that the system's inclination varies with time and that a central eclipse occurred ~1912, and we concur with Mossakovskaya that eclipses effectively ceased ~1940. Estimates of SS Lac on plates taken at Tashkent between 1937 and 1940 further serve to confirm the result. Thus, SS Lac belongs to a small but elite class of triple systems in which changes due to dynamical effects can be seen over a single human lifetime. In order to explore the properties of the SS Lac system, recent radial velocity curves and archival photographic and visual light curves have been analyzed with versions of the Wilson-Devinney code, augmented with a simplex routine to test solution uniqueness. The modeling solutions for the Dugan-Wright light curves ostensibly indicate that the former eclipsing system is composed of two early A stars of only slightly differing masses (2.57 ± 0.16 and 2.59 ± 0.19 M⊙) and effective surface temperatures (8750 ± 300 [assumed for component 1] and 8542 ± 309 K), but significantly different radii (2.38 ± 0.02 and 3.63 ± 0.07 R⊙) and luminosities (30 ± 4 and 63 ± 9 L⊙) for the hotter and cooler components, respectively. The light-curve solutions are compromised somewhat by variable eclipse depths over the ranges of dates of the data sets. This is especially true of the most complete light curve, that of Dugan & Wright; the others also suffer from incompleteness (that of Wachmann) and high scatter (that of Kordylewski, Pagaczewski, & Szafraniec). As a consequence, small, temporal variations in such system properties as the eccentricity, argument of periastron, modified Roche potentials, luminosities, and third light level, cannot be ruled out from currently available data. However, solutions with WD95, a self-iterating, damped–least squares version of the Wilson-Devinney program, reveal optimized inclinations for the data sets that project an inclination variation of 016 yr-1, but no evidence of apsidal motion. We find a distance for the system of 898 ± 95 pc, consistent with the value of Vansevi?ius et al. of 1040 ± 10 pc, and finally, on the bases of location on the sky, proper motion, radial velocity, photometry, and properties deduced in the present study, we confirm its membership in the cluster NGC 7209.
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