  V1741 Sgr (= SPICY 71482/Gaia22dtk) is a Classical T Tauri star on the
outskirts of the Lagoon Nebula. After at least a decade of stability, in
mid-2022, the optical source brightened by ~3 mag over two months, remained
bright until early 2023, then dimmed erratically over the next four months.
This event was monitored with optical and infrared spectroscopy and photometry.
Spectra from the peak (October 2022) indicate an EX Lup-type (EXor) accretion
outburst, with strong emission from H I, He I, and Ca II lines and CO bands. At
this stage, spectroscopic absorption features indicated a temperature of T ~
4750 K with low-gravity lines (e.g., Ba II and Sr II). By April 2023, with the
outburst beginning to dim, strong TiO absorption appeared, indicating a cooler
T ~ 3600 K temperature. However, once the source had returned to its
pre-outburst flux in August 2023, the TiO absorption and the CO emission
disappeared. When the star went into outburst, the source's spectral energy
distribution became flatter, leading to bluer colours at wavelengths shorter
than ~1.6 microns and redder colours at longer wavelengths. The brightening
requires a continuum emitting area larger than the stellar surface, likely from
optically thick circumstellar gas with cooler surface layers producing the
absorption features. Additional contributions to the outburst spectrum may
include blue excess from hotspots on the stellar surface, emission lines from
diffuse gas, and reprocessed emission from the dust disc. Cooling of the
circumstellar gas would explain the appearance of TiO, which subsequently
disappeared once this gas had faded and the stellar spectrum reemerged.
