  The Nuclear Spectroscopic Telescope Array (NuSTAR) provides an improvement in
sensitivity at energies above 10 keV by two orders of magnitude over
non-focusing satellites, making it possible to probe deeper into the Galaxy and
Universe. Lansbury and collaborators recently completed a catalog of 497
sources serendipitously detected in the 3-24 keV band using 13 deg2 of NuSTAR
coverage. Here, we report on an optical and X-ray study of 16 Galactic sources
in the catalog. We identify eight of them as stars (but some or all could have
binary companions), and use information from Gaia to report distances and X-ray
luminosities for three of them. There are four CVs or CV candidates, and we
argue that NuSTAR J233426-2343.9 is a relatively strong CV candidate based
partly on an X-ray spectrum from XMM-Newton. NuSTAR J092418-3142.2, which is
the brightest serendipitous source in the Lansbury catalog, and NuSTAR
J073959-3147.8 are LMXB candidates, but it is also possible that these two
sources are CVs. One of the sources is a known HMXB, and NuSTAR J105008-5958.8
is a new HMXB candidate, which has strong Balmer emission lines in its optical
spectrum and a hard X-ray spectrum. We discuss the implications of finding
these HMXBs for the surface density (logN-logS) and luminosity function of
Galactic HMXBs. We conclude that, with the large fraction of unclassified
sources in the Galactic plane detected by NuSTAR in the 8-24 keV band, there
could be a significant population of low luminosity HMXBs.
