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The chameleonic behaviour of the String theory dilaton is suggested. Some ofthe possible consequences of the chameleonic string dilaton are analyzed indetail. In particular, (1) we suggest a new stringy solution to thecosmological constant problem and (2) we point out the non-equivalence ofdifferent conformal frames at the quantum level. In order to obtain theseresults, we start taking into account the (strong coupling) string loopexpansion in the string frame (S-frame), therefore the so-called form factorsare present in the effective action. The correct Dark Energy scale is recoveredin the Einstein frame (E-frame) without unnatural fine-tunings and this resultis robust against all quantum corrections, granted that we assume a properstructure of the S-frame form factors in the strong coupling regime. At thisstage, the possibility still exists that a certain amount of fine-tuning may berequired to satisfy some phenomenological constraints. Moreover in the E-frame,in our proposal, all the interactions are switched off on cosmological lengthscales (i.e. the theory is IR-free), while higher derivative gravitationalterms might be present locally (on short distances) and it remains to be seenwhether these facts clash with phenomenology. A detailed phenomenologicalanalysis is definitely necessary to clarify these points.

Chameleonic dilaton, nonequivalent frames, and the cosmological constant problem in quantum string theory