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Early B-type main-sequence (MS) stars (M$_1$ = 5-16 M$_{\odot}$) with closelyorbiting low-mass stellar companions (q = M$_2$/M$_1$ < 0.25) can evolve toproduce Type Ia supernovae, low-mass X-ray binaries, and millisecond pulsars.However, the formation mechanism and intrinsic frequency of such close extrememass-ratio binaries have been debated, especially considering none havehitherto been detected. Utilizing observations of the Large Magellanic Cloudgalaxy conducted by the Optical Gravitational Lensing Experiment, we havediscovered a new class of eclipsing binaries in which a luminous B-type MS starirradiates a closely orbiting low-mass pre-MS companion that has not yet fullyformed. The primordial pre-MS companions have large radii and discerniblyreflect 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-MSeclipsing binaries in our sample with good model fits to the observed lightcurves, 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 stillassociated with stellar nurseries, e.g. the system with the deepest eclipse$\Delta$I$_1$ = 2.8 mag and youngest age $\tau$ = 0.6$\pm$0.4 Myr is embeddedin the bright H II region 30 Doradus. After correcting for selection effects,we find that (2.0$\pm$0.6)% of B-type MS stars have companions with shortorbital periods P = 3.0-8.5 days and extreme mass ratios q = 0.06-0.25. This is$\approx$10 times greater than that observed for solar-type MS primaries. Wediscuss how these new eclipsing binaries provide invaluable insights,diagnostics, and challenges for the formation and evolution of stars, binaries,and H II regions.

A New Class of Nascent Eclipsing Binaries with Extreme Mass Ratios