  Most millisecond pulsars with low-mass companions are in systems with either
helium-core white dwarfs or non-degenerate ("black widow" or "redback") stars.
A candidate counterpart to PSR J1816+4510 was identified by Kaplan et al.
(2012) whose properties were suggestive of both types of companions although
identical to neither. We have assembled optical spectroscopy of the candidate
companion and confirm that it is part of the binary system with a radial
velocity amplitude of 343+/-7 km/s, implying a high pulsar mass,
Mpsr*sin^3i=1.84+/-0.11 Msun, and a companion mass Mc*sin^3i=0.192+/-0.012
Msun, where i is the inclination of the orbit. The companion appears similar to
proto-white dwarfs/sdB stars, with a gravity log(g)=4.9+/-0.3, and effective
temperature 16000+/-500 K. The strongest lines in the spectrum are from
hydrogen, but numerous lines from helium, calcium, silicon, and magnesium are
present as well, with implied abundances of roughly ten times solar (relative
to hydrogen). As such, while from the spectrum the companion to PSR J1816+4510
is superficially most similar to a low-mass white dwarf, it has much lower
gravity, is substantially larger, and shows substantial metals. Furthermore, it
is able to produce ionized gas eclipses, which had previously been seen only
for low-mass, non-degenerate companions in redback or black widow systems. We
discuss the companion in relation to other sources, but find we understand
neither its nature nor its origins. Thus, the system is interesting for
understanding unusual stellar products of binary evolution, as well as,
independent of its nature, for determining neutron-star masses.
