UBV photometry of the ring in SW Cygni

Abstract

We present new UBV photometry of SW Cyg obtained in 1967, 1968, and 1969. Recent times of minimum yield a new ephemeris which suggests the period has not varied appreciably between 1964 and 1969. The bottom of primary eclipse exhibits several complications: asymmetries, nonconstant light between second and third contact, and an apparent ultraviolet excess in the cool subgiant of 8( U- B) = 0"'3. The most satisfactory explanation for this excess in SW Cyg (and presumably in most Algol-like binaries) is contamination by light from an envelope surrounding the hot star that is not completely eclipsed at midprimary minimum. From the shape of the 1968 primary eclipse we construct a map of the distribution of light from this envelope. The light on one side, by far the strongest in U, is a consequence of the gas strearn which must be striking the hot star on its trailing side. The light on the other side, however, has the color of the hot star and we suggest must originate from a protuberance in the photosphere of the hot star. We then show that the many photometric complications observed recently by Walter in his complete BV light curve of SW Cyg can be explained by our envelope-protuberance medel. Key words: eclipsing binaries - UBV photometry - photometric index anomalies