  We show that if there is a realistic 4-d string, the dilaton and moduli
supermultiplets will generically acquire a small mass O(m_{3/2}), providing the
only vacuum-independent evidence of low-energy physics in string theory beyond
the supersymmetric standard model. The only assumptions behind this result are
(i) softly broken supersymmetry at low energies with zero cosmological
constant, (ii) these particles interact with gravitational strength and the
scalar components have a flat potential in perturbation theory, which are
well-known properties of string theories. (iii) They acquire a $vev$ of the
order of the Planck scale (as required for the correct value of the gauge
coupling constants and the expected compactification scale) after supersymmetry
gets broken. We explore the cosmological implications of these particles.
Similar to the gravitino, the fermionic states may overclose the Universe if
they are stable or destroy nucleosynthesis if they decay unless their masses
belong to a certain range or inflation dilutes them. For the scalar states it
is known that the problem cannot be entirely solved by inflation, since
oscillations around the minimum of the potential can lead to a huge entropy
generation at late times. We discus some possible ways to alleviate this
entropy problem, that favour low-temperature baryogenesis, and also comment on
the possible role of these particles as dark matter candidates or as sources of
the baryon asymmetry through their decay.
