Gauge Coupling Unification: Strings versus SUSY-GUTs

Standard (level one) heterotic string models with standard model gauge group predict the unification of SU(3) and SU(2) gauge couplings whereas the U(1) factor is unified modulo an unknown normalization factor k1. On the other hand the unification mass is known. I argue that this situation is quite analogous (though opposite) to that in SUSYGUTs in which the U(1) normalization is known (k1 = 5/3) but the unification mass MX is unknown. I emphasize that k1 should be taken as a free parameter in the string approach (quite in the same way as MX is taken as a free parameter in SUSY-GUTs). If this is done, the success of the string approach concerning gauge coupling unification is comparable to that in SUSY-GUTs