  In this article we show that finite perturbative corrections in
non-supersymmetric strings can be understood via an interplay between modular
invariance and misaligned supersymmetry. While modular invariance is known to
be crucial in closed-string models, its presence and role for open strings is
more subtle. Nevertheless, we argue that it leads to cancellations in physical
quantities such as the one-loop cosmological constant and prevents them from
diverging. In particular, we show that if the sector-averaged number of states
does not grow exponentially, as predicted by misaligned supersymmetry, all
exponential divergences in the one-loop cosmological constant cancel out as
well. To account for the absence of power-law divergences, instead, we need to
resort to the modular structure of the partition function. We finally comment
on the presence of misaligned supersymmetry in the known 10-dimensional
tachyon-free non-supersymmetric string theories.
