  Single-line spectroscopic binaries recently contribute to the stellar-mass
black hole discovery, independently of the X-ray transient method. We report
the identification of a single-line binary system LTD064402+245919, with an
orbital period of 14.50 days. The observed component is a subgiant with a mass
of 2.77$\pm$0.68M$_{\odot}$, radius 15.5$\pm$2.5R$_{\odot}$, effective
temperature $T_{\rm eff}$ 4500$\pm$200K, and surface gravity log\emph{g}
2.5$\pm$0.25dex. The discovery makes use of the LAMOST time-domain (LAMOST-TD)
and ZTF survey. Our general-purpose software pipeline applies the Lomb-Scargle
periodogram to determine the orbital period and uses machine-learning to
classify the variable type from the folded light curves. We apply a combined
model to estimate the orbital parameters from both the light and radial
velocity curves, taking constraints on the primary star mass, mass function,
and detection limit of secondary luminosity into consideration. We obtain a
radial velocity semi-amplitude of 44.6$\pm$1.5 km s$^{-1}$, mass ratio of
0.73$\pm$0.07, and an undetected component mass of 2.02$\pm$0.49M$_{\odot}$
when the type of the undetected component is not set. We conclude that the
inclination is not well constrained, and that the secondary mass is larger than
1M$_{\odot}$ when the undetected component is modelled as a compact object.
According to our investigations using an MCMC simulation, increasing the
spectra SNR by a factor of 3 would enable the secondary light to be
distinguished (if present). The algorithm and software in this work are able to
serve as general-purpose tools for the identification of compact objects
quiescent in X-rays.
