Determining the Characteristics of Lan 30 from Optical Observations

We have performed a model analysis of the optical radiation from the young pre-cataclysmic variable Lan 30 and determined the set of its fundamental characteristics. Spectroscopic and multiband photometric observations have been carried out with the BTA and Zeiss-1000 telescopes at the Special Astrophysical Observatory of the Russian Academy of Sciences. The technique for modeling irradiated atmospheres of stars in close binary systems has been used for the computations of synthetic spectra and light curves. We have established the dominance of the radiation from an sdB subdwarf with atmospheric parameters $$T_{\textrm{eff}}=30\,500\pm 1100$$ K and $$\log g=5.60\pm 0.10$$ in the optical band under moderate influence of reflection effects on the light curves and HI line profiles. We have analyzed the measured sets of radial velocities of the primary component by taking into account the possible influence of reflection effects on them and refined their semi-amplitude $$K_{1}=56.3\pm 2.0$$ km s $${}^{-1}$$ . We have found that the nearly sinusoidal shape of the light curves of Lan 30 allows only an upper limit on the orbital inclination to be set. Based on agreement between the model and observed light curves, we have estimated the radii of the components at various inclinations. To determine their lower limit, we have used evolutionary estimates of the radii of low-mass main-sequence stars. The mass of the primary component has been taken to be equal to the normal mass of single sdB subdwarfs, $$M_{1}=0.47M_{\odot}$$ . As a result, we have determined the ranges of possible variations of the orbital inclination $$i=33^{\circ}{-}45^{\circ}$$ and semimajor axis, the radii of the components, and the mass $$M_{2}=0.127{-}0.175M_{\odot}$$ of the red dwarf.