The supergiant eclipsing system BL Telescopii

The long period (778d) eclipsing system BL Tel is discussed on the basis of the spectroscopic orbit, the light curve and high dispersion spectra inside and outside eclipse. The supergiant F-type primary (mass zoO Mv = - 8m.0 R1 3000 radii) has normal (solar) abundances (with the possible exception of europium). The primary undergoes an annular eclipse by a slightly smaller and very much fainter body. The spectroscopic changes observed during eclipse are chiefly due to the centre to limb variations in the primary. The secondary (mass 70 R2 = ?i) fills its lobe of the critical equipotential surface through the inner Lagrangian point. It may either be a cool supergiant or a hot (subdwarf) star ionizing a small, dense, H II region which produces the eclipse. The high velocity of the system and its great distance from the galactic plane (z = 3'3 kpc) classifies the object as a `run-away' system. The present state of the system is satisfied by the assumption that the secondary was originally a very massive star ( 4000) and that at some time I years ago) it lost most of its mass very rapidly, probably as a result of a type II supernova outburst. The principal numerical data for the system are listed in Table I.