Second Outburst of the Yellow Symbiotic Star LT Delphini

Abstract

We present the results of our photoelectric UBV observations of the yellow symbiotic star LT Del over 2010-2018. The binary system LT Del, which consists of a bright K giant and a compact hot star with a temperature of 100 000 K, has an orbital period of 476 days. In 2017 the variable experienced a second low-amplitude (Δ V 0 ^ṃ 7) outburst in the history of its studies whose maximum occurred at an orbital phase of 0.15±0.05. The outburst duration was 60 days. The B-V and U-B colors in the outburst became noticeably bluer. A difference in the photometric behavior of the star in the 1994 and 2017 outburst has been detected. In the orbital cycle preceding the 2017 outburst a secondary minimum with a depth of 0 ^ṃ 7 and 0 ^ṃ 20 appeared in the B and V light curves, respectively, whose cause is discussed. The phase light and color curves are presented and explained; the position of the star on the color-color diagram is interpreted. We have estimated the parameters of the cool and hot components of the system based on the distance determination from Gaia DR2.