Symbiotic halo star LT Del: Phase variations of the emission spectrum and parameters of the cool component

Long-term photometric and spectroscopic observations of the yellow symbiotic star LT Del are analyzed. UBV light curves are presented. Based on the observations of 20 cycles, we have refined the orbital period of the star, P = 476{_·/^d}0 ± 1{_·/^d}0. The brightness has been found to be unstable at some orbital phases with an amplitude up to 0{_·/ ^m } 3. We have measured the fluxes in hydrogen and helium emission lines and in continuum and investigated their relationship to the orbital period. The fluxes in hydrogen and HeI lines follow the UBV light curves in phase; the He II 4686 Å flux does not depend on the phase and is constant within the accuracy of our measurements. The intensity ratio of the 4686 Å andH β lines changes from 0.2 to 0.9 over the period. We interpret the spectroscopic observations based on the hypothesis of heating and ionization of the stellar wind from a cool component by high-frequency radiation from a hot star with a temperature of 105 K. We have estimated the spectral type of the cool star from our photometry and its continuum energy distribution as a bright K2-4 red giant branch halo star. The bolometric luminosity and mass loss rate have been estimated for the K component to be L _bol ∼ 700 L _⊙ and dot{M} ∼ 10^-8 M _⊙ yr^-1, respectively.