# Second Outburst of the Yellow Symbiotic Star LT Delphini

## TL;DR

This study reports on the 2017 outburst of the yellow symbiotic star LT Del, analyzing its photometric behavior, color changes, and system parameters, revealing differences from previous outbursts and providing insights into its binary nature.

## Contribution

First detailed photometric analysis of the 2017 outburst of LT Del, including color behavior, orbital phase specifics, and system parameter estimation based on Gaia data.

## Key findings

- 2017 outburst lasted about 60 days with a bluer color during maximum.
- Detected a secondary minimum in the orbital cycle before the outburst.
- Estimated system parameters using Gaia DR2 distance data.

## 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 $\sim$100 000 K, has an orbital period of 476 days. In 2017 the variable experienced a second low-amplitude ($\Delta V\sim 0.^{m}7$) outburst in the history of its studies whose maximum occurred at an orbital phase of 0.15$\pm$0.05. The outburst duration was $\sim$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 $0.^{m}15$ and $0.^{m}20$ 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.