4 Draconis : a unique triple system containing an M3 giant and a cataclysmic binary.

Abstract

The M3 III giant 4 Dra has a UV bright companion discovered with IUE as a probable cataclysmic variable (CV). The authors determine the orbit of the M giant around the centre of gravity of the system that it forms with the cataclysmic binary and find P = 1703 d, e = 0.30, a₁msin i = 82 Gm, and f(m) = 0.0076 M_sun;. They find that 4 Dra B shows spectrum variations like a CV and determine the photometric period as P = 3^h58^m.5±1^m. 4 Dra B is the first CV for which an independent distance estimate (≅100 pc) can be given from the spectroscopic parallax of the companion. Properties of the CV, which is probably of the magnetic type (AM Her), are briefly discussed. The age of the CV may be less than 10⁸yr.