Eclipsing Binaries – full information
Algol – the “demon” star
The shape and timing of the eclipses gives the shape and size of the stars.
You know the system is nearly edge on. Since you also know the velocity (from Doppler shift) and orbital period, you can get the true scale of the system (including star sizes and masses). The separation has to be pretty small for the odds to be good this will happen.