  Black holes are now commonplace, among the stars, in Galactic centers, and
perhaps other places. But within living memory, their very existence was
doubted by many, and few chose to look for them. Zeldovich and Guseinov were
first, followed by Trimble and Thorne, using a method that would have
identified HDE 226868 as a plausible candidate, if it had been in the 1968
catalogue of spectroscopic binaries. That it was not arose from an unhappy
accident in the observing program of Daniel M. Popper long before the discovery
of X-ray binaries and the identification of Cygnus X-1 with that hot, massive
star and its collapsed companion.
