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The early B-type eclipsing binary GT\,Cephei: a massive triple system?

GT\,Cep is a semi-detached close binary system with an orbital period of 4.91 days, containing a massive star. I have obtained spectroscopic observations and derived radial velocities of both components. Combining the analyses of radial velocities and available photometric observations we have measured the absolute parameters of both components of GT\,Cep. The components are shown to be a B2\,V primary with a mass M$_p$=10.70$\pm$0.50 M$_{\odot}$ and radius R$_p$=6.83$\pm$0.19 R$_{\odot}$ and a A0\,IV secondary with a mass M$_s$=2.58$\pm$0.14 M$_{\odot}$ and radius R$_s$=7.56$\pm$0.21 R$_{\odot}$. My analyses show that GT\,Cep is a classical Algol-type binary with a less massive secondary filling its $Roche$ lobe. Using the UBVJHK magnitudes and the interstellar reddening of E(B-V)=0.61 I estimated the mean distance to the system as 854$\pm$43\,pc. The O-C residuals have been analyzed as the consequence of a light-time effect superimposed on an upward parabola. My analysis indicates that the eclipsing binary revolves around a third-body with a period of about 57.5 yr in an orbit with a radius of 40 $AU$. The lower limit for the mass of the third star has been estimated to be 7 M$_{\odot}$ for the inclination between $70^{o}$ and $90^{o}$.