Moduli Stars
  We explore the possibility that (Bose-Einstein) condensation of scalar fields
from string compactifications can lead to long-lived compact objects. Depending
on the type of scalar fields we find different realisations of star-like and
solitonic objects. We illustrate in the framework of type~IIB string
compactifications that closed string moduli can lead to heavy microscopic stars
with masses of order $\mathcal{V}^\alpha M_{\rm Planck}$, $\alpha=1,3/2,5/3$
where $\mathcal{V}$ is the volume of the extra dimensions. Macroscopic compact
objects from ultra-light string axions are realised with masses of order
$e^{\mathcal{V}^{2/3}}M_{\rm Planck}.$ Q-ball configurations can be obtained
from open string moduli whereas the closed string sector gives rise to a new
class of solutions, named PQ-balls, that arise in the two-field axion-modulus
system. The stability, the potential for the formation, and the observability
of moduli stars through gravitational waves are discussed. In particular we
point out that during the early matter phase given by moduli domination,
density perturbations grow by a factor $\mathcal{V}^{\beta}$ with $\beta>2$ and
non-linear effects cannot be neglected.
