  Current theories of massless free particle assume {\sl unitary} space
inversion and {\sl anti-unitary} time reversal operators. In so doing robust
classes of possible theories are discarded. In the present work theories of
massless systems are derived through a strictly deductive development from the
principle of relativistic invariance, so that a kind of space inversion or time
reversal operator is ruled out only if it causes inconsistencies. As results,
new classes of consistent theories for massless isolated systems are explicitly
determined. On the other hand, the approach determines definite constraints
implied by the invariance principle; they were ignored by some past
investigations that, as a consequence, turn out to be not consistent with the
invariance principle. Also the problem of the localizability for massless
systems is reconsidered within the new theoretical framework, obtaining a
generalization and a deeper detailing of previous results.
