An optical and near-infrared study of the AM Herculis-type binary CW 1103+254

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Abstract
We have observed the AM Hercuiis-type binary CW 1103+254 using optical and near-infrared photometry and polarimetry, and optical spectroscopy and spectropolarimetry. We find that from such a set of observations it is relatively easy to distinguish all the main components of the system. Faint-phase infrared radiation comes predominantly from the M-dwarf secondary. We use the infrared magnitude of the secondary to estimate the distance of the system as 136 pc. From ellipsoidal variations of this component we find that the orbital period does not differ from the white-dwarf rotation period by more than one part in 60 000. The observed emission line profiles can be understood in terms of two components, one originating in the stream of material infalling onto the white dwarf, while the other originates in the vicinity of the secondary. Faint-phase optical radiation originates in the white-dwarf photosphere and shows Zeeman features corresponding to a magnetic field of 18 MG. The excess radiation during the bright phase can be understood as cyclotron radiation from the shock-heated region near the white-dwarf magnetic pole. In the infrared where the cyclotron region is optically thick, radiation at different wavelengths originates at different heights in the shock-heated region, and shows that the temperature of the region does not change much with height. Changes in the system between 1982 and 1983 indicate precessional motion of the white-dwarf rotation or magnetic axis.

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