  A star expands to become a red giant when it has fused all the hydrogen in
its core into helium. If the star is in a binary system, its envelope can
overflow onto its companion or be ejected into space, leaving a hot core and
potentially forming a subdwarf-B star. However, most red giants that have
partially transferred envelopes in this way remain cool on the surface and are
almost indistinguishable from those that have not. Among $\sim$7000
helium-burning red giants observed by NASA's Kepler mission, we use
asteroseismology to identify two classes of stars that must have undergone
dramatic mass loss, presumably due to stripping in binary interactions. The
first class comprises about 7 underluminous stars with smaller helium-burning
cores than their single-star counterparts. Theoretical models show that these
small cores imply the stars had much larger masses when ascending the red giant
branch. The second class consists of 32 red giants with masses down to 0.5
M$_\odot$, whose implied ages would exceed the age of the universe had no mass
loss occurred. The numbers are consistent with binary statistics, and our
results open up new possibilities to study the evolution of post-mass-transfer
binary systems.
