  We report two binary systems, LAMOST J035540+381550 (hereafter J035540) and
LAMOST J035916+400732 (hereafter J035916), identified through the Large Sky
Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) low-resolution survey
(LRS). Each of these two systems contains an M-type star orbiting with a
invisible compact object candidate. Follow-up spectroscopic observations of
Palomar 200-inch telescope (P200) enhance radial velocity measurements. We use
radial velocities from LAMOST and P200, as well as light curves from Zwicky
Transient Facility (ZTF) to constrain orbital parameters. The masses of the
visible M-type stars are estimated by fitting the MIST isochrones and SEDs. The
mass functions for the unseen companions are: $0.22\pm 0.01 M_{\odot}$ for
J035540 and $0.16\pm 0.01 M_{\odot}$ for J035916. With the orbital and stellar
parameters derived above and assuming the orbital inclination is 90 degree
(edge-on), we find that the minimum masses of the invisible companions exceeds
that of the visible stars. The single-lined feature and the dynamical evidence
suggest the presence of compact objects. J035540's ZTF light curve, modeled
with PHOEBE, yields a compact object mass of $0.70^{+0.12}_{-0.05} M_{\odot}$.
For J035916, ellipsoidal modulation analysis constrains the light curve
amplitude, yielding a compact object mass range of $0.57-0.90 M_{\odot}$. The
mass estimates indicate that both are likely white dwarfs. These findings
underscore the efficiency of optical time-domain surveys and dynamical methods
in identifying faint, massive white dwarfs, along with other compact objects in
binaries.
