Analysis of the U-band orbital variation of the symbiotic binary AG Draconis during quiescence

The photometric U orbital variations of the symbiotic binary AG Dra, caused by occultation of a bright gaseous region by the K giant in this system, are used to derive the emission from the circumbinary nebula, that is supposed to be generated by the cool giant’s wind. Assuming spherical symmetry and constant expansion velocity for the wind, and that the giant has the same intrinsic continuum energy distribution of α Boo, we derive a cool giant’s radius of 28÷32 R and a distance of 1560÷1810 pc. We conclude that the cool giant does not fill its Roche lobe, which is contrasting with the intense, accretion powered X-ray emission.