  Double white-dwarf (DWD) mergers are relevant astrophysical sources expected
to produce massive, highly-magnetized WDs, supernovae (SNe) Ia, and neutron
stars (NSs). Although they are expected to be numerous sources in the sky,
their detection has evaded the most advanced transient surveys. This article
characterizes the optical transient expected from DWD mergers in which the
central remnant is a stable (sub-Chandrasekhar) WD. We show that the expansion
and cooling of the merger's dynamical ejecta lead to an optical emission
peaking at $1$-$10$ d post-merger, with luminosities of $10^{40}$-$10^{41}$ erg
s$^{-1}$. We present simulations of the light-curves, spectra, and the color
evolution of the transient. We show that these properties, together with the
estimated rate of mergers, are consistent with the absence of detection, e.g.,
by The Zwicky Transient Facility (ZTF). More importantly, we show that the
Legacy Survey of Space and Time (LSST) of the Vera C. Rubin Observatory will
likely detect a few/several hundred per year, opening a new window to the
physics of WDs, NSs, and SN Ia.
