Ramond-Ramond photons are a generic feature of type

II string compactifications. Being typically massless and gravitationally coupled

to string moduli, those hidden photons behave as dark radiation produced at

reheating from the decay of the lightest modulus. In this talk I will inspect

their production mechanism and I will show that the requirement to avoid dark

radiation overproduction poses a strong challenge for any model which features

flat complex structure moduli at tree-level, as argued by the tadpole conjecture.

We will discover how the best case scenario is the one where all complex structure

moduli are heavy and reheating is driven by the decay of the Kähler moduli

whose decay width into RR hidden photons is negligible.