  We present results of a Hubble Space Telescope far-ultraviolet (FUV) survey
searching for white dwarf (WD) companions to blue straggler stars (BSSs) in
open cluster NGC 188. The majority of NGC 188 BSSs (15 of 21) are single-lined
binaries with properties suggestive of mass-transfer formation via Roche lobe
overflow, specifically through an asymptotic giant branch star transferring
mass to a main sequence secondary, yielding a BSS binary with a WD companion.
In NGC 188, a BSS formed by this mechanism within the past 400 Myr will have a
WD companion hot and luminous enough to be directly detected as a FUV
photometric excess with HST. Comparing expected BSS FUV emission to observed
photometry reveals four BSSs with WD companions above 12,000 K (younger than
250 Myr) and three WD companions with temperatures between 11,000-12,000 K.
These BSS+WD binaries all formed through recent mass transfer. The location of
the young BSSs in an optical color-magnitude diagram (CMD) indicates that
distance from the zero-age main sequence does not necessarily correlate with
BSS age. There is no clear CMD separation between mass transfer-formed BSSs and
those likely formed through other mechanisms, such as collisions. The seven
detected WD companions place a lower limit on the mass-transfer formation
frequency of 33%. We consider other possible formation mechanisms by comparing
properties of the BSS population to theoretical predictions. We conclude that
14 BSS binaries likely formed from mass transfer, resulting in an inferred
mass-transfer formation frequency of approximately 67%.
