Light curve analysis and parameter study of the EB-type eclipsing binary V 836 Cygni

The EB-type short-period eclipsing binary V836 Cygni has been analysed on the basis of earlier published and hitherto unexploited fairly extensive UBV photometry. The Simplex method for parameter optimization, applied to the Wilson–Devinney model approach, proved a very effective tool to determine the best-fit solution. The corresponding global minimum of σfit, however, is only marginally pronounced, and thereby allows the light curve to be fitted over a whole range of parameters (with a mass-ratio interval |$0.3\le q\le 0.5$|⁠). This indeterminacy, likely to result from the low secondary light level relative to the observational noise, probably explains the discrepancies of previous photometric and spectroscopic analyses. It is partly overcome by evaluating the four-colour Strömgren observations contained in the photometric binary-star survey of Hilditch & Hill, which determine, through comparison with the calibrated synthetic photometric grids of Lester, Gray & Kurucz and in combination with the mass function, a fairly accurate parameter set and indicate a low mass ratio near q ≈ 0.34. The system thus appears to have evolved already through the stage of rapid mass exchange with mass-ratio reversal, as the primary component is an almost normal A0 V star, whilst the G-type secondary, oversized and therefore overluminous compared to a main-sequence star of its surface brightness, is in marginal contact with its Roche lobe.