  Understanding ionizing fluxes of stellar populations is crucial for various
astrophysical problems including the epoch of reionization. Massive short-lived
Wolf-Rayet stars are generally considered as the main ionizing sources. We
examine the role of less massive stars that lose their envelope through
interaction with a companion. We use the evolutionary code MESA and the
radiative transfer code CMFGEN to investigate stripped stars as a function of
metallicity (Z).
  We show that typical progenitors, initially 12$M_{\odot}$, produce hot and
compact stars (~4$M_{\odot}$, 60-80 kK, ~1$R_{\odot}$) that copiously produce
ionizing photons, emitting 60-85% and 30-60% of their energy as HI and HeI
ionizing radiation, for Z=0.0001-0.02. This is comparable to a typical massive
Wolf-Rayet star, when accounting for their longer lifetimes and the favoring
initial mass function. Stripped stars further emit their ionizing photons with
a delay (~20Myrs after star formation for this progenitor), allowing time for
stellar feedback to disperse the birth clouds. This increases the fraction that
can escape and contribute to ionization of the intergalactic medium.
  We further find that Roche stripping fails to fully remove the H-rich
envelope at low Z. This questions the common treatment of stripped stars in
rapid population synthesis simulations as pure helium stars. We expect
implications for the rate of type Ib/c supernova at low Z and for the advanced
evolutionary channels for type Ia supernova and gravitational wave sources. We
discuss how the characteristic spectral features of stripped stars can be used
to increase the observed sample, which is urgently needed to test the models.
