Spectrum of the Yellow Symbiotic Star LT Delphini before, during, and after the 2017 Outburst

LT Del is a yellow symbiotic system that consists of a bright K3-type giant and a hot subdwarf with a temperature of about 10$^5$ K. We present the results of our spectroscopic observations of LT Del over the period 2010-2018. In 2017 the star experienced a second low-amplitude ($\Delta V\sim0.^{m}7$) outburst in the history of its studies. The emission spectrum of the star represented in the optical range by hydrogen, neutral and ionized helium lines underwent significant changes in the outburst. The fluxes in the HI and HeI emission lines increased by a factor of 5-6, the HeII $\lambda$4686 line grew by a factor of 10. According to our estimates, in the 2017 outburst, the temperature of the exciting star rose to $T_{\text{hot}}\sim$130~000 K, while during the first 1994 outburst the change in temperature was insignificant. This suggests cool and hot outbursts of LT Del by analogy with similar events of another yellow symbiotic star, AG Dra.