We report on detailed studies of the absorption line spectrum and spectroscopic orbit of the binary BN2.5Ib star HD 235679. We also give ao preliminary report on our study of the Hα emission line profiles from this perplexing system. Hipparcos and Tycho2 astrometric data allow us to place limits on the distance to the system. The lack of a measurable reflection effect in the Hipparcos photometry allows us to rule out the possibility that the massive invisible star is cool and fills its Roche lobe. Thus, by process of elimination, the invisible star must be hot or a black hole. The properties of the Hα profiles suggest that the invisible star is somewhat hotter, and has a stronger wind than HD235679.
FOTEL 4 is a FORTRAN code for separate or simultaneous solving of light curves, radial-velocity curves, visual (interferometric) measurements and eclipse timing of binary and/or triple stellar systems. The underlying physical assumptions, numerical methods and practical use of the code are described in this document.
The code KOREL for Fourier disentangling (i.e. simultaneous decomposition of component spectra and solution of orbital elements) and lime-photometry of binary and multiple stars is described together with auxiliary codes PREKOR, KORNOR etc.