Early B-type main-sequence ( MS ) stars ( M _ { 1 } \approx 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 _ { refl } \approx 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 \approx 0.6 - 8 Myr , and small secondary masses M _ { 2 } \approx 0.8 - 2.4 M _ { \odot } ( q \approx 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 \approx 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 .