The radii of the components of 31 Cyg.

The eclipsing binary 31 Cyg has components of spectral type K3.5Ib and B4V. The supergiant consists of a star of almost sharp edge and an extended atmosphere. For the light curve of the primary minimum the eclipsing effect of the extended atmosphere has been removed and the radii of the components determined. The photometric elements have been computed in case of an eclipsing star with a sharp edge and also for a star where the optical depth along the line of sight has been determined from model atmosphere calculations. In case of central eclipse the radius of the B star is found equal to 7.2 B , which is a value too large for the spectral type B4V. In case of a noncentral eclipse we find smaller values of the B star radius. Due to the uncertainty of the orbit dimensions and assumptions made, a definite value of the orbital inclination is not presented but it is concluded that the eclipse may not necessarily be central. The equations valid for an atmospheric eclipse are presented and explained in detail. Key words: eclipsing binary - photometric element determination - atmospheric eclipse