What is the minimum mass of a neutron star?

[Vanadium 50]

I don't have my copy of Shapiro & Teukolsky handy

I do. They give 0.18 solar masses under Harrison-Wheeler EOS. The Oppenheimer and Volkoff EOS has no lower bound. The reason they don't spring back to white dwarfs is, as you say, that this requires work, and the energy to do that work has already left the system.

 

[GeorgeDishman]

I've found this paper which probably meets my needs:

https://iopscience.iop.org/article/10.1088/0004-637X/778/1/66
Equation (3) gives a range at formation for an isolated NS as:

M_birth∼1.08–1.57M_☉

Presumably a star exceeding the Chandrasekhar limit would start to collapse but the implosion of the core could eject some surface material to leave a remnant slightly below the limit.

In binary systems, mass transfer raises the mass of the first NS formed as the companion sheds so they have a different distribution with a higher mean (see figure 2) but I don't see any way the mass could be reduced significantly after formation.