A photometric and spectroscopic study of the symbiotic binary BF Cyg

Abstract

We present the historical 1890-1996 photographiclB-band light curve of the symbiotic binary BF Cyg for the first time. It displays three types of eruption: (i) a slow, symbiotic nova outburst (1895-1960), probably caused by a thermonuclear event; (ii) short-term flares dependent on the orbital phase, possibly triggered by accretion events, and (iii) outbursts (1920, 1989) such as are observed in classical symbiotic stars. Analyses of the deep minimum in the optical continuum during the last outburst, the observed bolo metric luminosity of the cool component, and elements of the hot component orbit [Kh =25.9 km s-l,fh(m) = 1.4 M0 ] indicate the system to be 4.6-5.4 kpc distant, and show that the binary consists of an M bright giant (Mg ' 2 M0 Rg~240-280 R 0 Lg~5000 L 0 ), filling up or being close to its Roche lobe, and a hot, luminous, compact object (Mh , 0.3-0.4 M0 Lh ~ 1.4Lg ) with an orbital inclination i~70-90°. Our parameters are very different from those previously published.