  Early B-type main-sequence (MS) stars (M$_1$ = 5-16 M$_{\odot}$) with closely
orbiting low-mass stellar companions (q = M$_2$/M$_1$ < 0.25) can evolve to
produce Type Ia supernovae, low-mass X-ray binaries, and millisecond pulsars.
However, the formation mechanism and intrinsic frequency of such close extreme
mass-ratio binaries have been debated, especially considering none have
hitherto been detected. Utilizing observations of the Large Magellanic Cloud
galaxy conducted by the Optical Gravitational Lensing Experiment, we have
discovered a new class of eclipsing binaries in which a luminous B-type MS star
irradiates a closely orbiting low-mass pre-MS companion that has not yet fully
formed. The primordial pre-MS companions have large radii and discernibly
reflect much of the light they intercept from the B-type MS primaries
($\Delta$I$_{\rm refl}$ = 0.02-0.14 mag). For the 18 definitive MS + pre-MS
eclipsing binaries in our sample with good model fits to the observed light
curves, we measure short orbital periods P = 3.0-8.5 days, young ages $\tau$ =
0.6-8 Myr, and small secondary masses M$_2$ = 0.8-2.4 M$_{\odot}$ (q =
0.07-0.36). The majority of these nascent eclipsing binaries are still
associated with stellar nurseries, e.g. the system with the deepest eclipse
$\Delta$I$_1$ = 2.8 mag and youngest age $\tau$ = 0.6$\pm$0.4 Myr is embedded
in the bright H II region 30 Doradus. After correcting for selection effects,
we find that (2.0$\pm$0.6)% of B-type MS stars have companions with short
orbital periods P = 3.0-8.5 days and extreme mass ratios q = 0.06-0.25. This is
$\approx$10 times greater than that observed for solar-type MS primaries. We
discuss how these new eclipsing binaries provide invaluable insights,
diagnostics, and challenges for the formation and evolution of stars, binaries,
and H II regions.
