{\it Ab--initio} finite temperature excitons

The coupling with the lattice vibrations is shown to drastically modify the state--of--the--art picture of the excitonic states based on a frozen atom approximation. The zero--point vibrations renormalize the bare energies and optical strengths. Excitons acquire a non--radiative lifetime that decreases with increasing temperature. The optical brightness and efficiency turn out to be strongly temperature dependent such as to induce bright to dark (and vice versa) transitions. The finite temperature experimental optical absorption spectra of bulk Si and hexagonal BN are successfully explained without using any external parameter.