  We present and discuss the results of our photometric and spectroscopic
monitoring of 2SXPS J173508.4-292958 carried out from April to August 2020.
This X-ray source, in the foreground with respect to the Galactic center,
brightened in X-rays during 2020, prompting our follow-up optical observations.
We found the star to contain a K4III giant with a modest but highly variable
Halpha emission, composed by a ~470 km/s wide component with superimposed a
narrow absorption, offset by a positive velocity with respect to the giant. No
orbital motion is detected for the K4III, showing an heliocentric radial
velocity stable at -12(+/-1) km/s. No flickering in excess of 0.005 mag in B
band was observed at three separate visits of 2SXPS J173508.4-292958. While
photometrically stable in 2016 through 2018, in 2019 the star developed a
limited photometric variability, that in 2020 took the form of a sinusoidal
modulation with a period of 38 days and an amplitude of 0.12 mag in V band. We
argue this variability cannot be ascribed to Roche-lobe filling by the K4III
star. No correlation is observed between the photometric variability and the
amount of emission in Halpha, the latter probably originating directly from the
accretion disk around the accreting companion. While no emission from dust is
detected at mid-IR wavelengths, an excess in U-band is probably present and
caused by direct emission from the accretion disk. We conclude that 2SXPS
J173508.4-292958 is a new symbiotic star of the accreting-only variety
(AO-SySt).

Solar and Stellar Astrophysics