Weighing the string mass with the COBE data

In the context of the pre-big bang scenario the large-scale CMB anisotropy can be seeded by a primordial background of very light (or massless) axion fluctuations. In that case the slope of the temperature anisotropy spectrum, allowed by present observations, defines an allowed range of values for the string mass scale. Conversely, from the theoretical expected value of the string scale we can predict the slope of the anisotropy spectrum. In both cases there is a remarkable agreement between observations and theoretical expectations.